hgweb: send proper HTTP response after uncaught exception

This patch fixes a bug where hgweb would send an incomplete HTTP
response.

If an uncaught exception is raised when hgweb is processing a request,
hgweb attempts to send a generic error response and log that exception.

The server defaults to chunked transfer coding. If an uncaught exception
occurred, it was sending the error response string / chunk properly.
However, RFC 7230 Section 4.1 mandates a 0 size last chunk be sent to
indicate end of the entity body. hgweb was failing to send this last
chunk. As a result, properly written HTTP clients would assume more data
was coming and they would likely time out waiting for another chunk to
arrive.

Mercurial's own test harness was paving over the improper HTTP behavior
by not attempting to read the response body if the status code was 500.
This incorrect workaround was added in ba6577a19656 and has been removed
with this patch.

tests/hgweberror.py

@@ -0,0 +1,17 b''
+# A dummy extension that installs an hgweb command that throws an Exception.
+
+from mercurial.hgweb import webcommands
+
+def raiseerror(web, req, tmpl):
+    '''Dummy web command that raises an uncaught Exception.'''
+
+    # Simulate an error after partial response.
+    if 'partialresponse' in req.form:
+        req.respond(200, 'text/plain')
+        req.write('partial content\n')
+
+    raise AttributeError('I am an uncaught error!')
+
+def extsetup(ui):
+    setattr(webcommands, 'raiseerror', raiseerror)
+    webcommands.__all__.append('raiseerror')

mercurial/hgweb/server.py

@@ -81,6 +81,7 b' class _httprequesthandler(BaseHTTPServer'
         except Exception:
             self._start_response("500 Internal Server Error", [])
             self._write("Internal Server Error")
+            self._done()
             tb = "".join(traceback.format_exception(*sys.exc_info()))
             self.log_error("Exception happened during processing "
                            "request '%s':\n%s", self.path, tb)

tests/get-with-headers.py

@@ -43,9 +43,8 b' def request(host, path, show):'
             print "%s: %s" % (h, response.getheader(h))
     if not headeronly:
         print
-        if response.status != 500:
-            data = response.read()
-            sys.stdout.write(data)
+        data = response.read()
+        sys.stdout.write(data)
 
         if twice and response.getheader('ETag', None):
             tag = response.getheader('ETag')

tests/test-hgweb.t

@@ -579,4 +579,30 b' errors'
 
   $ cat errors.log
 
+Uncaught exceptions result in a logged error and canned HTTP response
+
+  $ "$TESTDIR/killdaemons.py" $DAEMON_PIDS
+  $ hg --config extensions.hgweberror=$TESTDIR/hgweberror.py serve -p $HGPORT -d --pid-file=hg.pid -A access.log -E errors.log
+  $ cat hg.pid >> $DAEMON_PIDS
+
+  $ $TESTDIR/get-with-headers.py localhost:$HGPORT 'raiseerror' transfer-encoding content-type
+  500 Internal Server Error
+  transfer-encoding: chunked
+  
+  Internal Server Error (no-eol)
+  [1]
+
+  $ head -1 errors.log
+  .* Exception happened during processing request '/raiseerror': (re)
+
+Uncaught exception after partial content sent
+
+  $ $TESTDIR/get-with-headers.py localhost:$HGPORT 'raiseerror?partialresponse=1' transfer-encoding content-type
+  200 Script output follows
+  transfer-encoding: chunked
+  content-type: text/plain
+  
+  partial content
+  Internal Server Error (no-eol)
+
   $ cd ..