[PATCH] http-backend: fix die recursion with custom handler

On Wed, May 13, 2015 at 08:47:24PM -0400, Jeff King wrote:

There's a minor bug in git's error reporting that makes this a little
harder to examine, but isn't the root cause. I'll send a patch for that
momentarily.  But here's what I've found.

Here's that patch. You may have seen "recursion detected in die handler"
in your apache logs.  Basically we die(), try to write an HTTP error
response, and then die() trying to write it again. It happens reliably
here because this particular error happens _after_ we've already written
out the normal response header and closed stdout. So any die() we
encounter after that is going to try to write its own error header, and
will fail because of the closed stdout.

One way of avoiding this would obviously be to notice in the die()
handler that we have already written our header and closed stdout. That
would help this case, but it would not help any other case where writing
fails unexpectedly. So I'd rather solve the general recursion problem,
which covers all cases.

-- >8 --
Subject: http-backend: fix die recursion with custom handler

When we die() in http-backend, we call a custom handler that
writes an HTTP 500 response to stdout, then reports the
error to stderr. Our routines for writing out the HTTP
response may themselves die, leading to us entering die()
again.

When it was originally written, that was OK; our custom
handler keeps a variable to notice this and does not recurse
indefinitely. However, since cd163d4 (usage.c: detect
recursion in die routines and bail out immediately,
2012-11-14), the main die() implementation detects recursion
before we even get to our custom handler, and bails without
printing anything useful.

We can handle this case by doing two things:

  1. Installing a custom die_is_recursing handler that
     allows us to enter up to one level of recursion. Only
     the first call to our custom handler will try to write
     out the error response. So if we die again, that is OK.
     If we end up dying more than that, it is a sign that we
     have a bug and are in an infinite recursion (i.e., what
     cd163d4 was designed to protect against).

  2. Reporting the error to stderr before trying to write
     out the HTTP response. In the current code, if we do
     die() trying to write out the response, we'll exit
     immediately from this second die(), and never get a
     chance to output the original error (which is almost
     certainly the more interesting one; the second die is
     just going to be along the lines of "I tried to write
     to stdout but it was closed").

Signed-off-by: Jeff King <redacted>
---
 http-backend.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/http-backend.c b/http-backend.c
index b6c0484..c210d4d 100644
--- a/http-backend.c
+++ b/http-backend.c

@@ -500,21 +500,25 @@ static void service_rpc(char *service_name)
 	strbuf_release(&buf);
 }
 
+static int dead;
 static NORETURN void die_webcgi(const char *err, va_list params)
 {
-	static int dead;
+	if (dead <= 1) {
+		vreportf("fatal: ", err, params);
 
-	if (!dead) {
-		dead = 1;
 		http_status(500, "Internal Server Error");
 		hdr_nocache();
 		end_headers();
-
-		vreportf("fatal: ", err, params);
 	}
 	exit(0); /* we successfully reported a failure ;-) */
 }
 
+static int die_webcgi_recursing(void)
+{
+	dead++;
+	return dead > 1;
+}
+
 static char* getdir(void)
 {
 	struct strbuf buf = STRBUF_INIT;

@@ -569,6 +573,7 @@ int main(int argc, char **argv)
 
 	git_extract_argv0_path(argv[0]);
 	set_die_routine(die_webcgi);
+	set_die_is_recursing_routine(die_webcgi_recursing);
 
 	if (!method)
 		die("No REQUEST_METHOD from server");

-- 
2.4.0.327.ge28c153