GIT_TRACE: show which built-in/external commands are executed

With the environment variable GIT_TRACE set git will show
 - alias expansion
 - built-in command execution
 - external command execution
on stderr.

Signed-off-by: Matthias Lederhofer <matled@gmx.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git.txt

@@ -615,6 +615,13 @@ git Diffs
 	gitlink:git-diff-files[1];
 	gitlink:git-diff-tree[1]
 
+other
+~~~~~
+'GIT_TRACE'::
+	If this variable is set git will print `trace:` messages on
+	stderr telling about alias expansion, built-in command
+	execution and external command execution.
+
 Discussion[[Discussion]]
 ------------------------
 include::README[]

exec_cmd.c

@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "exec_cmd.h"
+#include "quote.h"
 #define MAX_ARGS	32
 
 extern char **environ;
@@ -96,9 +97,27 @@ int execv_git_cmd(const char **argv)
 		tmp = argv[0];
 		argv[0] = git_command;
 
+		if (getenv("GIT_TRACE")) {
+			fputs("trace: exec:", stderr);
+			const char **p = argv;
+			while (*p) {
+				fputc(' ', stderr);
+				sq_quote_print(stderr, *p);
+				++p;
+			}
+			putc('\n', stderr);
+			fflush(stderr);
+		}
+
 		/* execve() can only ever return if it fails */
 		execve(git_command, (char **)argv, environ);
 
+		if (getenv("GIT_TRACE")) {
+			fprintf(stderr, "trace: exec failed: %s\n",
+				strerror(errno));
+			fflush(stderr);
+		}
+
 		argv[0] = tmp;
 	}
 	return -1;

git.c

@@ -11,6 +11,7 @@
 #include "git-compat-util.h"
 #include "exec_cmd.h"
 #include "cache.h"
+#include "quote.h"
 
 #include "builtin.h"
 
@@ -120,6 +121,18 @@ static int handle_alias(int *argcp, const char ***argv)
 			if (!strcmp(alias_command, new_argv[0]))
 				die("recursive alias: %s", alias_command);
 
+			if (getenv("GIT_TRACE")) {
+				int i;
+				fprintf(stderr, "trace: alias expansion: %s =>",
+					alias_command);
+				for (i = 0; i < count; ++i) {
+					fputc(' ', stderr);
+					sq_quote_print(stderr, new_argv[i]);
+				}
+				fputc('\n', stderr);
+				fflush(stderr);
+			}
+
 			/* insert after command name */
 			if (*argcp > 1) {
 				new_argv = realloc(new_argv, sizeof(char*) *
@@ -202,6 +215,18 @@ static void handle_internal_command(int argc, const char **argv, char **envp)
 		struct cmd_struct *p = commands+i;
 		if (strcmp(p->cmd, cmd))
 			continue;
+
+		if (getenv("GIT_TRACE")) {
+			int i;
+			fprintf(stderr, "trace: built-in: git");
+			for (i = 0; i < argc; ++i) {
+				fputc(' ', stderr);
+				sq_quote_print(stderr, argv[i]);
+			}
+			putc('\n', stderr);
+			fflush(stderr);
+		}
+
 		exit(p->fn(argc, argv, envp));
 	}
 }

quote.c

@@ -45,6 +45,23 @@ size_t sq_quote_buf(char *dst, size_t n, const char *src)
 	return len;
 }
 
+void sq_quote_print(FILE *stream, const char *src)
+{
+	char c;
+
+	fputc('\'', stream);
+	while ((c = *src++)) {
+		if (need_bs_quote(c)) {
+			fputs("'\\", stream);
+			fputc(c, stream);
+			fputc('\'', stream);
+		} else {
+			fputc(c, stream);
+		}
+	}
+	fputc('\'', stream);
+}
+
 char *sq_quote(const char *src)
 {
 	char *buf;

quote.h

@@ -29,6 +29,7 @@
  */
 
 extern char *sq_quote(const char *src);
+extern void sq_quote_print(FILE *stream, const char *src);
 extern size_t sq_quote_buf(char *dst, size_t n, const char *src);
 
 /* This unwraps what sq_quote() produces in place, but returns