trace2/tr2_dst.c - jrn/git - Git at Google

 #include "cache.h"
 #include "sigchain.h"
 #include "trace2/tr2_dst.h"
 #include "trace2/tr2_sid.h"
 #include "trace2/tr2_sysenv.h"

 /*
  * How many attempts we will make at creating an automatically-named trace file.
  */
 #define MAX_AUTO_ATTEMPTS 10

 /*
  * Sentinel file used to detect when we should discard new traces to avoid
  * writing too many trace files to a directory.
  */
 #define DISCARD_SENTINEL_NAME "git-trace2-discard"

 /*
  * When set to zero, disables directory file count checks. Otherwise, controls
  * how many files we can write to a directory before entering discard mode.
  * This can be overridden via the TR2_SYSENV_MAX_FILES setting.
  */
 static int tr2env_max_files = 0;

 static int tr2_dst_want_warning(void)
 {
 	static int tr2env_dst_debug = -1;

 	if (tr2env_dst_debug == -1) {
 		const char *env_value = tr2_sysenv_get(TR2_SYSENV_DST_DEBUG);
 		if (!env_value || !*env_value)
 			tr2env_dst_debug = 0;
 		else
 			tr2env_dst_debug = atoi(env_value) > 0;
 	}

 	return tr2env_dst_debug;
 }

 void tr2_dst_trace_disable(struct tr2_dst *dst)
 {
 	if (dst->need_close)
 		close(dst->fd);
 	dst->fd = 0;
 	dst->initialized = 1;
 	dst->need_close = 0;
 }

 /*
  * Check to make sure we're not overloading the target directory with too many
  * files. First get the threshold (if present) from the config or envvar. If
  * it's zero or unset, disable this check. Next check for the presence of a
  * sentinel file, then check file count.
  *
  * Returns 0 if tracing should proceed as normal. Returns 1 if the sentinel file
  * already exists, which means tracing should be disabled. Returns -1 if there
  * are too many files but there was no sentinel file, which means we have
  * created and should write traces to the sentinel file.
  *
  * We expect that some trace processing system is gradually collecting files
  * from the target directory; after it removes the sentinel file we'll start
  * writing traces again.
  */
 static int tr2_dst_too_many_files(struct tr2_dst *dst, const char *tgt_prefix)
 {
 	int file_count = 0, max_files = 0, ret = 0;
 	const char *max_files_var;
 	DIR *dirp;
 	struct strbuf path = STRBUF_INIT, sentinel_path = STRBUF_INIT;
 	struct stat statbuf;

 	/* Get the config or envvar and decide if we should continue this check */
 	max_files_var = tr2_sysenv_get(TR2_SYSENV_MAX_FILES);
 	if (max_files_var && *max_files_var && ((max_files = atoi(max_files_var)) >= 0))
 		tr2env_max_files = max_files;

 	if (!tr2env_max_files) {
 		ret = 0;
 		goto cleanup;
 	}

 	strbuf_addstr(&path, tgt_prefix);
 	if (!is_dir_sep(path.buf[path.len - 1])) {
 		strbuf_addch(&path, '/');
 	}

 	/* check sentinel */
 	strbuf_addbuf(&sentinel_path, &path);
 	strbuf_addstr(&sentinel_path, DISCARD_SENTINEL_NAME);
 	if (!stat(sentinel_path.buf, &statbuf)) {
 		ret = 1;
 		goto cleanup;
 	}

 	/* check file count */
 	dirp = opendir(path.buf);
 	while (file_count < tr2env_max_files && dirp && readdir(dirp))
 		file_count++;
 	if (dirp)
 		closedir(dirp);

 	if (file_count >= tr2env_max_files) {
 		dst->too_many_files = 1;
 		dst->fd = open(sentinel_path.buf, O_WRONLY | O_CREAT | O_EXCL, 0666);
 		ret = -1;
 		goto cleanup;
 	}

 cleanup:
 	strbuf_release(&path);
 	strbuf_release(&sentinel_path);
 	return ret;
 }

 static int tr2_dst_try_auto_path(struct tr2_dst *dst, const char *tgt_prefix)
 {
 	int too_many_files;
 	const char *last_slash, *sid = tr2_sid_get();
 	struct strbuf path = STRBUF_INIT;
 	size_t base_path_len;
 	unsigned attempt_count;

 	last_slash = strrchr(sid, '/');
 	if (last_slash)
 		sid = last_slash + 1;

 	strbuf_addstr(&path, tgt_prefix);
 	if (!is_dir_sep(path.buf[path.len - 1]))
 		strbuf_addch(&path, '/');
 	strbuf_addstr(&path, sid);
 	base_path_len = path.len;

 	too_many_files = tr2_dst_too_many_files(dst, tgt_prefix);
 	if (!too_many_files) {
 		for (attempt_count = 0; attempt_count < MAX_AUTO_ATTEMPTS; attempt_count++) {
 			if (attempt_count > 0) {
 				strbuf_setlen(&path, base_path_len);
 				strbuf_addf(&path, ".%d", attempt_count);
 			}

 			dst->fd = open(path.buf, O_WRONLY | O_CREAT | O_EXCL, 0666);
 			if (dst->fd != -1)
 				break;
 		}
 	} else if (too_many_files == 1) {
 		strbuf_release(&path);
 		if (tr2_dst_want_warning())
 			warning("trace2: not opening %s trace file due to too "
 				"many files in target directory %s",
 				tr2_sysenv_display_name(dst->sysenv_var),
 				tgt_prefix);
 		return 0;
 	}

 	if (dst->fd == -1) {
 		if (tr2_dst_want_warning())
 			warning("trace2: could not open '%.*s' for '%s' tracing: %s",
 				(int) base_path_len, path.buf,
 				tr2_sysenv_display_name(dst->sysenv_var),
 				strerror(errno));

 		tr2_dst_trace_disable(dst);
 		strbuf_release(&path);
 		return 0;
 	}

 	strbuf_release(&path);

 	dst->need_close = 1;
 	dst->initialized = 1;

 	return dst->fd;
 }

 static int tr2_dst_try_path(struct tr2_dst *dst, const char *tgt_value)
 {
 	int fd = open(tgt_value, O_WRONLY | O_APPEND | O_CREAT, 0666);
 	if (fd == -1) {
 		if (tr2_dst_want_warning())
 			warning("trace2: could not open '%s' for '%s' tracing: %s",
 				tgt_value,
 				tr2_sysenv_display_name(dst->sysenv_var),
 				strerror(errno));

 		tr2_dst_trace_disable(dst);
 		return 0;
 	}

 	dst->fd = fd;
 	dst->need_close = 1;
 	dst->initialized = 1;

 	return dst->fd;
 }

 #ifndef NO_UNIX_SOCKETS
 #define PREFIX_AF_UNIX "af_unix:"
 #define PREFIX_AF_UNIX_STREAM "af_unix:stream:"
 #define PREFIX_AF_UNIX_DGRAM "af_unix:dgram:"

 static int tr2_dst_try_uds_connect(const char *path, int sock_type, int *out_fd)
 {
 	int fd;
 	struct sockaddr_un sa;

 	fd = socket(AF_UNIX, sock_type, 0);
 	if (fd == -1)
 		return -1;

 	sa.sun_family = AF_UNIX;
 	strlcpy(sa.sun_path, path, sizeof(sa.sun_path));

 	if (connect(fd, (struct sockaddr *)&sa, sizeof(sa)) == -1) {
 		int saved_errno = errno;
 		close(fd);
 		errno = saved_errno;
 		return -1;
 	}

 	*out_fd = fd;
 	return 0;
 }

 #define TR2_DST_UDS_TRY_STREAM (1 << 0)
 #define TR2_DST_UDS_TRY_DGRAM  (1 << 1)

 static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
 					  const char *tgt_value)
 {
 	unsigned int uds_try = 0;
 	int fd;
 	const char *path = NULL;

 	/*
 	 * Allow "af_unix:[<type>:]<absolute_path>"
 	 *
 	 * Trace2 always writes complete individual messages (without
 	 * chunking), so we can talk to either DGRAM or STREAM type sockets.
 	 *
 	 * Allow the user to explicitly request the socket type.
 	 *
 	 * If they omit the socket type, try one and then the other.
 	 */

 	if (skip_prefix(tgt_value, PREFIX_AF_UNIX_STREAM, &path))
 		uds_try |= TR2_DST_UDS_TRY_STREAM;

 	else if (skip_prefix(tgt_value, PREFIX_AF_UNIX_DGRAM, &path))
 		uds_try |= TR2_DST_UDS_TRY_DGRAM;

 	else if (skip_prefix(tgt_value, PREFIX_AF_UNIX, &path))
 		uds_try |= TR2_DST_UDS_TRY_STREAM | TR2_DST_UDS_TRY_DGRAM;

 	if (!path || !*path) {
 		if (tr2_dst_want_warning())
 			warning("trace2: invalid AF_UNIX value '%s' for '%s' tracing",
 				tgt_value,
 				tr2_sysenv_display_name(dst->sysenv_var));

 		tr2_dst_trace_disable(dst);
 		return 0;
 	}

 	if (!is_absolute_path(path) ||
 	    strlen(path) >= sizeof(((struct sockaddr_un *)0)->sun_path)) {
 		if (tr2_dst_want_warning())
 			warning("trace2: invalid AF_UNIX path '%s' for '%s' tracing",
 				path, tr2_sysenv_display_name(dst->sysenv_var));

 		tr2_dst_trace_disable(dst);
 		return 0;
 	}

 	if (uds_try & TR2_DST_UDS_TRY_STREAM) {
 		if (!tr2_dst_try_uds_connect(path, SOCK_STREAM, &fd))
 			goto connected;
 		if (errno != EPROTOTYPE)
 			goto error;
 	}
 	if (uds_try & TR2_DST_UDS_TRY_DGRAM) {
 		if (!tr2_dst_try_uds_connect(path, SOCK_DGRAM, &fd))
 			goto connected;
 	}

 error:
 	if (tr2_dst_want_warning())
 		warning("trace2: could not connect to socket '%s' for '%s' tracing: %s",
 			path, tr2_sysenv_display_name(dst->sysenv_var),
 			strerror(errno));

 	tr2_dst_trace_disable(dst);
 	return 0;

 connected:
 	dst->fd = fd;
 	dst->need_close = 1;
 	dst->initialized = 1;

 	return dst->fd;
 }
 #endif

 static void tr2_dst_malformed_warning(struct tr2_dst *dst,
 				      const char *tgt_value)
 {
 	warning("trace2: unknown value for '%s': '%s'",
 		tr2_sysenv_display_name(dst->sysenv_var), tgt_value);
 }

 int tr2_dst_get_trace_fd(struct tr2_dst *dst)
 {
 	const char *tgt_value;

 	/* don't open twice */
 	if (dst->initialized)
 		return dst->fd;

 	dst->initialized = 1;

 	tgt_value = tr2_sysenv_get(dst->sysenv_var);

 	if (!tgt_value || !strcmp(tgt_value, "") || !strcmp(tgt_value, "0") ||
 	    !strcasecmp(tgt_value, "false")) {
 		dst->fd = 0;
 		return dst->fd;
 	}

 	if (!strcmp(tgt_value, "1") || !strcasecmp(tgt_value, "true")) {
 		dst->fd = STDERR_FILENO;
 		return dst->fd;
 	}

 	if (strlen(tgt_value) == 1 && isdigit(*tgt_value)) {
 		dst->fd = atoi(tgt_value);
 		return dst->fd;
 	}

 	if (is_absolute_path(tgt_value)) {
 		if (is_directory(tgt_value))
 			return tr2_dst_try_auto_path(dst, tgt_value);
 		else
 			return tr2_dst_try_path(dst, tgt_value);
 	}

 #ifndef NO_UNIX_SOCKETS
 	if (starts_with(tgt_value, PREFIX_AF_UNIX))
 		return tr2_dst_try_unix_domain_socket(dst, tgt_value);
 #endif

 	/* Always warn about malformed values. */
 	tr2_dst_malformed_warning(dst, tgt_value);
 	tr2_dst_trace_disable(dst);
 	return 0;
 }

 int tr2_dst_trace_want(struct tr2_dst *dst)
 {
 	return !!tr2_dst_get_trace_fd(dst);
 }

 void tr2_dst_write_line(struct tr2_dst *dst, struct strbuf *buf_line)
 {
 	int fd = tr2_dst_get_trace_fd(dst);
 	ssize_t bytes;

 	strbuf_complete_line(buf_line); /* ensure final NL on buffer */

 	/*
 	 * We do not use write_in_full() because we do not want
 	 * a short-write to try again.  We are using O_APPEND mode
 	 * files and the kernel handles the atomic seek+write. If
 	 * another thread or git process is concurrently writing to
 	 * this fd or file, our remainder-write may not be contiguous
 	 * with our initial write of this message.  And that will
 	 * confuse readers.  So just don't bother.
 	 *
 	 * It is assumed that TRACE2 messages are short enough that
 	 * the system can write them in 1 attempt and we won't see
 	 * a short-write.
 	 *
 	 * If we get an IO error, just close the trace dst.
 	 */
 	sigchain_push(SIGPIPE, SIG_IGN);
 	bytes = write(fd, buf_line->buf, buf_line->len);
 	sigchain_pop(SIGPIPE);
 	if (bytes >= 0)
 		return;

 	tr2_dst_trace_disable(dst);
 	if (tr2_dst_want_warning())
 		warning("unable to write trace to '%s': %s",
 			tr2_sysenv_display_name(dst->sysenv_var),
 			strerror(errno));
 }
	#include "cache.h"
	#include "sigchain.h"
	#include "trace2/tr2_dst.h"
	#include "trace2/tr2_sid.h"
	#include "trace2/tr2_sysenv.h"

	/*
	* How many attempts we will make at creating an automatically-named trace file.
	*/
	#define MAX_AUTO_ATTEMPTS 10

	/*
	* Sentinel file used to detect when we should discard new traces to avoid
	* writing too many trace files to a directory.
	*/
	#define DISCARD_SENTINEL_NAME "git-trace2-discard"

	/*
	* When set to zero, disables directory file count checks. Otherwise, controls
	* how many files we can write to a directory before entering discard mode.
	* This can be overridden via the TR2_SYSENV_MAX_FILES setting.
	*/
	static int tr2env_max_files = 0;

	static int tr2_dst_want_warning(void)
	{
	static int tr2env_dst_debug = -1;

	if (tr2env_dst_debug == -1) {
	const char *env_value = tr2_sysenv_get(TR2_SYSENV_DST_DEBUG);
	if (!env_value \|\| !*env_value)
	tr2env_dst_debug = 0;
	else
	tr2env_dst_debug = atoi(env_value) > 0;
	}

	return tr2env_dst_debug;
	}

	void tr2_dst_trace_disable(struct tr2_dst *dst)
	{
	if (dst->need_close)
	close(dst->fd);
	dst->fd = 0;
	dst->initialized = 1;
	dst->need_close = 0;
	}

	/*
	* Check to make sure we're not overloading the target directory with too many
	* files. First get the threshold (if present) from the config or envvar. If
	* it's zero or unset, disable this check. Next check for the presence of a
	* sentinel file, then check file count.
	*
	* Returns 0 if tracing should proceed as normal. Returns 1 if the sentinel file
	* already exists, which means tracing should be disabled. Returns -1 if there
	* are too many files but there was no sentinel file, which means we have
	* created and should write traces to the sentinel file.
	*
	* We expect that some trace processing system is gradually collecting files
	* from the target directory; after it removes the sentinel file we'll start
	* writing traces again.
	*/
	static int tr2_dst_too_many_files(struct tr2_dst dst, const char tgt_prefix)
	{
	int file_count = 0, max_files = 0, ret = 0;
	const char *max_files_var;
	DIR *dirp;
	struct strbuf path = STRBUF_INIT, sentinel_path = STRBUF_INIT;
	struct stat statbuf;

	/* Get the config or envvar and decide if we should continue this check */
	max_files_var = tr2_sysenv_get(TR2_SYSENV_MAX_FILES);
	if (max_files_var && *max_files_var && ((max_files = atoi(max_files_var)) >= 0))
	tr2env_max_files = max_files;

	if (!tr2env_max_files) {
	ret = 0;
	goto cleanup;
	}

	strbuf_addstr(&path, tgt_prefix);
	if (!is_dir_sep(path.buf[path.len - 1])) {
	strbuf_addch(&path, '/');
	}

	/* check sentinel */
	strbuf_addbuf(&sentinel_path, &path);
	strbuf_addstr(&sentinel_path, DISCARD_SENTINEL_NAME);
	if (!stat(sentinel_path.buf, &statbuf)) {
	ret = 1;
	goto cleanup;
	}

	/* check file count */
	dirp = opendir(path.buf);
	while (file_count < tr2env_max_files && dirp && readdir(dirp))
	file_count++;
	if (dirp)
	closedir(dirp);

	if (file_count >= tr2env_max_files) {
	dst->too_many_files = 1;
	dst->fd = open(sentinel_path.buf, O_WRONLY \| O_CREAT \| O_EXCL, 0666);
	ret = -1;
	goto cleanup;
	}

	cleanup:
	strbuf_release(&path);
	strbuf_release(&sentinel_path);
	return ret;
	}

	static int tr2_dst_try_auto_path(struct tr2_dst dst, const char tgt_prefix)
	{
	int too_many_files;
	const char last_slash, sid = tr2_sid_get();
	struct strbuf path = STRBUF_INIT;
	size_t base_path_len;
	unsigned attempt_count;

	last_slash = strrchr(sid, '/');
	if (last_slash)
	sid = last_slash + 1;

	strbuf_addstr(&path, tgt_prefix);
	if (!is_dir_sep(path.buf[path.len - 1]))
	strbuf_addch(&path, '/');
	strbuf_addstr(&path, sid);
	base_path_len = path.len;

	too_many_files = tr2_dst_too_many_files(dst, tgt_prefix);
	if (!too_many_files) {
	for (attempt_count = 0; attempt_count < MAX_AUTO_ATTEMPTS; attempt_count++) {
	if (attempt_count > 0) {
	strbuf_setlen(&path, base_path_len);
	strbuf_addf(&path, ".%d", attempt_count);
	}

	dst->fd = open(path.buf, O_WRONLY \| O_CREAT \| O_EXCL, 0666);
	if (dst->fd != -1)
	break;
	}
	} else if (too_many_files == 1) {
	strbuf_release(&path);
	if (tr2_dst_want_warning())
	warning("trace2: not opening %s trace file due to too "
	"many files in target directory %s",
	tr2_sysenv_display_name(dst->sysenv_var),
	tgt_prefix);
	return 0;
	}

	if (dst->fd == -1) {
	if (tr2_dst_want_warning())
	warning("trace2: could not open '%.*s' for '%s' tracing: %s",
	(int) base_path_len, path.buf,
	tr2_sysenv_display_name(dst->sysenv_var),
	strerror(errno));

	tr2_dst_trace_disable(dst);
	strbuf_release(&path);
	return 0;
	}

	strbuf_release(&path);

	dst->need_close = 1;
	dst->initialized = 1;

	return dst->fd;
	}

	static int tr2_dst_try_path(struct tr2_dst dst, const char tgt_value)
	{
	int fd = open(tgt_value, O_WRONLY \| O_APPEND \| O_CREAT, 0666);
	if (fd == -1) {
	if (tr2_dst_want_warning())
	warning("trace2: could not open '%s' for '%s' tracing: %s",
	tgt_value,
	tr2_sysenv_display_name(dst->sysenv_var),
	strerror(errno));

	tr2_dst_trace_disable(dst);
	return 0;
	}

	dst->fd = fd;
	dst->need_close = 1;
	dst->initialized = 1;

	return dst->fd;
	}

	#ifndef NO_UNIX_SOCKETS
	#define PREFIX_AF_UNIX "af_unix:"
	#define PREFIX_AF_UNIX_STREAM "af_unix:stream:"
	#define PREFIX_AF_UNIX_DGRAM "af_unix:dgram:"

	static int tr2_dst_try_uds_connect(const char path, int sock_type, int out_fd)
	{
	int fd;
	struct sockaddr_un sa;

	fd = socket(AF_UNIX, sock_type, 0);
	if (fd == -1)
	return -1;

	sa.sun_family = AF_UNIX;
	strlcpy(sa.sun_path, path, sizeof(sa.sun_path));

	if (connect(fd, (struct sockaddr *)&sa, sizeof(sa)) == -1) {
	int saved_errno = errno;
	close(fd);
	errno = saved_errno;
	return -1;
	}

	*out_fd = fd;
	return 0;
	}

	#define TR2_DST_UDS_TRY_STREAM (1 << 0)
	#define TR2_DST_UDS_TRY_DGRAM (1 << 1)

	static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
	const char *tgt_value)
	{
	unsigned int uds_try = 0;
	int fd;
	const char *path = NULL;

	/*
	* Allow "af_unix:[<type>:]<absolute_path>"
	*
	* Trace2 always writes complete individual messages (without
	* chunking), so we can talk to either DGRAM or STREAM type sockets.
	*
	* Allow the user to explicitly request the socket type.
	*
	* If they omit the socket type, try one and then the other.
	*/

	if (skip_prefix(tgt_value, PREFIX_AF_UNIX_STREAM, &path))
	uds_try \|= TR2_DST_UDS_TRY_STREAM;

	else if (skip_prefix(tgt_value, PREFIX_AF_UNIX_DGRAM, &path))
	uds_try \|= TR2_DST_UDS_TRY_DGRAM;

	else if (skip_prefix(tgt_value, PREFIX_AF_UNIX, &path))
	uds_try \|= TR2_DST_UDS_TRY_STREAM \| TR2_DST_UDS_TRY_DGRAM;

	if (!path \|\| !*path) {
	if (tr2_dst_want_warning())
	warning("trace2: invalid AF_UNIX value '%s' for '%s' tracing",
	tgt_value,
	tr2_sysenv_display_name(dst->sysenv_var));

	tr2_dst_trace_disable(dst);
	return 0;
	}

	if (!is_absolute_path(path) \|\|
	strlen(path) >= sizeof(((struct sockaddr_un *)0)->sun_path)) {
	if (tr2_dst_want_warning())
	warning("trace2: invalid AF_UNIX path '%s' for '%s' tracing",
	path, tr2_sysenv_display_name(dst->sysenv_var));

	tr2_dst_trace_disable(dst);
	return 0;
	}

	if (uds_try & TR2_DST_UDS_TRY_STREAM) {
	if (!tr2_dst_try_uds_connect(path, SOCK_STREAM, &fd))
	goto connected;
	if (errno != EPROTOTYPE)
	goto error;
	}
	if (uds_try & TR2_DST_UDS_TRY_DGRAM) {
	if (!tr2_dst_try_uds_connect(path, SOCK_DGRAM, &fd))
	goto connected;
	}

	error:
	if (tr2_dst_want_warning())
	warning("trace2: could not connect to socket '%s' for '%s' tracing: %s",
	path, tr2_sysenv_display_name(dst->sysenv_var),
	strerror(errno));

	tr2_dst_trace_disable(dst);
	return 0;

	connected:
	dst->fd = fd;
	dst->need_close = 1;
	dst->initialized = 1;

	return dst->fd;
	}
	#endif

	static void tr2_dst_malformed_warning(struct tr2_dst *dst,
	const char *tgt_value)
	{
	warning("trace2: unknown value for '%s': '%s'",
	tr2_sysenv_display_name(dst->sysenv_var), tgt_value);
	}

	int tr2_dst_get_trace_fd(struct tr2_dst *dst)
	{
	const char *tgt_value;

	/* don't open twice */
	if (dst->initialized)
	return dst->fd;

	dst->initialized = 1;

	tgt_value = tr2_sysenv_get(dst->sysenv_var);

	if (!tgt_value \|\| !strcmp(tgt_value, "") \|\| !strcmp(tgt_value, "0") \|\|
	!strcasecmp(tgt_value, "false")) {
	dst->fd = 0;
	return dst->fd;
	}

	if (!strcmp(tgt_value, "1") \|\| !strcasecmp(tgt_value, "true")) {
	dst->fd = STDERR_FILENO;
	return dst->fd;
	}

	if (strlen(tgt_value) == 1 && isdigit(*tgt_value)) {
	dst->fd = atoi(tgt_value);
	return dst->fd;
	}

	if (is_absolute_path(tgt_value)) {
	if (is_directory(tgt_value))
	return tr2_dst_try_auto_path(dst, tgt_value);
	else
	return tr2_dst_try_path(dst, tgt_value);
	}

	#ifndef NO_UNIX_SOCKETS
	if (starts_with(tgt_value, PREFIX_AF_UNIX))
	return tr2_dst_try_unix_domain_socket(dst, tgt_value);
	#endif

	/* Always warn about malformed values. */
	tr2_dst_malformed_warning(dst, tgt_value);
	tr2_dst_trace_disable(dst);
	return 0;
	}

	int tr2_dst_trace_want(struct tr2_dst *dst)
	{
	return !!tr2_dst_get_trace_fd(dst);
	}

	void tr2_dst_write_line(struct tr2_dst dst, struct strbuf buf_line)
	{
	int fd = tr2_dst_get_trace_fd(dst);
	ssize_t bytes;

	strbuf_complete_line(buf_line); /* ensure final NL on buffer */

	/*
	* We do not use write_in_full() because we do not want
	* a short-write to try again. We are using O_APPEND mode
	* files and the kernel handles the atomic seek+write. If
	* another thread or git process is concurrently writing to
	* this fd or file, our remainder-write may not be contiguous
	* with our initial write of this message. And that will
	* confuse readers. So just don't bother.
	*
	* It is assumed that TRACE2 messages are short enough that
	* the system can write them in 1 attempt and we won't see
	* a short-write.
	*
	* If we get an IO error, just close the trace dst.
	*/
	sigchain_push(SIGPIPE, SIG_IGN);
	bytes = write(fd, buf_line->buf, buf_line->len);
	sigchain_pop(SIGPIPE);
	if (bytes >= 0)
	return;

	tr2_dst_trace_disable(dst);
	if (tr2_dst_want_warning())
	warning("unable to write trace to '%s': %s",
	tr2_sysenv_display_name(dst->sysenv_var),
	strerror(errno));
	}