trace: implement %p placeholder for filenames

This lets you trace output on a per-process basis by naming
the trace file after the PID. This is probably not all that
useful for GIT_TRACE itself (which is about a global view of
the process hierarchy anyway), but will be useful for traces
which produce large amounts of data (e.g., whole packfiles).

Signed-off-by: Jeff King <peff@peff.net>

Author: peff

Documentation/git.adoc

@@ -804,7 +804,8 @@ trace messages into this file descriptor.
 Alternatively, if the variable is set to an absolute path
 (starting with a '/' character), Git will interpret this
 as a file path and will try to append the trace messages
-to it.
+to it. If the filename contains the string `%p`, that string
+will be replaced with the PID of the traced process.
 +
 Unsetting the variable, or setting it to empty, "0" or
 "false" (case insensitive) disables trace messages.

trace.c

@@ -53,15 +53,28 @@ static int get_trace_fd(struct trace_key *key, const char *override_envvar)
 	else if (strlen(trace) == 1 && isdigit(*trace))
 		key->fd = atoi(trace);
 	else if (is_absolute_path(trace)) {
-		int fd = open(trace, O_WRONLY | O_APPEND | O_CREAT, 0666);
+		struct strbuf name = STRBUF_INIT;
+		int fd;
+
+		while (strbuf_expand_step(&name, &trace)) {
+			if (skip_prefix(trace, "%", &trace))
+				strbuf_addch(&name, '%');
+			else if (skip_prefix(trace, "p", &trace))
+				strbuf_addf(&name, "%lu", (unsigned long)getpid());
+			else
+				strbuf_addch(&name, '%');
+		}
+
+		fd = open(name.buf, O_WRONLY | O_APPEND | O_CREAT, 0666);
 		if (fd == -1) {
 			warning("could not open '%s' for tracing: %s",
-				trace, strerror(errno));
+				name.buf, strerror(errno));
 			trace_disable(key);
 		} else {
 			key->fd = fd;
 			key->need_close = 1;
 		}
+		strbuf_release(&name);
 	} else {
 		warning("unknown trace value for '%s': %s\n"
 			"         If you want to trace into a file, then please set %s\n"