run-command: Redirect stderr to a pipe before redirecting stdout to stderr

With this patch, in the 'start_command' function after forking
we now take care of stderr in the child process before stdout.

This way if 'start_command' is called with a 'child_process'
argument like this:

	.err = -1;
	.stdout_to_stderr = 1;

then stderr will be redirected to a pipe before stdout is
redirected to stderr. So we can now get the process' stdout
from the pipe (as well as its stderr).

Earlier such a call would have redirected stdout to stderr
before stderr was itself redirected, and therefore stdout
would not have followed stderr, which would not have been
very useful anyway.

Update documentation in 'api-run-command.txt' accordingly.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Acked-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 11 insertions, 10 deletions

diff --git a/Documentation/technical/api-run-command.txt b/Documentation/technical/api-run-command.txt
index fde3b4532..c364a22c8 100644
--- a/Documentation/technical/api-run-command.txt
+++ b/Documentation/technical/api-run-command.txt
@@ -111,9 +111,10 @@ stderr as follows:
 	.no_stdin, .no_stdout, .no_stderr: The respective channel is
 		redirected to /dev/null.
 
-	.stdout_to_stderr: stdout of the child is redirected to the
-		parent's stderr (i.e. *not* to what .err or
-		.no_stderr specify).
+	.stdout_to_stderr: stdout of the child is redirected to its
+		stderr. This happens after stderr is itself redirected.
+		So stdout will follow stderr to wherever it is
+		redirected.
 
 To modify the environment of the sub-process, specify an array of
 string pointers (NULL terminated) in .env:
diff --git a/run-command.c b/run-command.c
index 743757c36..44100a749 100644
--- a/run-command.c
+++ b/run-command.c
@@ -91,6 +91,13 @@ int start_command(struct child_process *cmd)
 			close(cmd->in);
 		}
 
+		if (cmd->no_stderr)
+			dup_devnull(2);
+		else if (need_err) {
+			dup2(fderr[1], 2);
+			close_pair(fderr);
+		}
+
 		if (cmd->no_stdout)
 			dup_devnull(1);
 		else if (cmd->stdout_to_stderr)
@@ -103,13 +110,6 @@ int start_command(struct child_process *cmd)
 			close(cmd->out);
 		}
 
-		if (cmd->no_stderr)
-			dup_devnull(2);
-		else if (need_err) {
-			dup2(fderr[1], 2);
-			close_pair(fderr);
-		}
-
 		if (cmd->dir && chdir(cmd->dir))
 			die("exec %s: cd to %s failed (%s)", cmd->argv[0],
 			    cmd->dir, strerror(errno));