CVS Server: Support reading base and roots from environment

The Gitosis single-account Git/ssh hosting system runs git commands
through git-shell after confirming that the connecting user is
authorized to access the requested repository. This works well for
upload-pack and receive-pack, which take a repository argument through
git-shell. This doesn't work so well for `cvs server', which is passed
through literally, with no arguments. Allowing arguments risks
sneaking in `--export-all', so that restriction should be maintained.

Despite that, passing a repository root is necessary for per-user
access control by the hosting software, and passing a base path
improves usability without weakening security. Thus, git-cvsserver
needs to come up with these values at runtime by some other
means. Since git-shell preserves the environment for other purposes,
the environment can carry these arguments as well.

Thus, modify git-cvsserver to read $GIT_CVSSERVER_{BASE_PATH,ROOT} in
the absence of equivalent command line arguments.

Signed-off-by: Phil Miller <mille121@illinois.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-cvsserver.txt

@@ -277,6 +277,21 @@ In `dbdriver` and `dbuser` you can use the following variables:
 	If no name can be determined, the
 	numeric uid is used.
 
+ENVIRONMENT
+-----------
+
+These variables obviate the need for command-line options in some
+circumstances, allowing easier restricted usage through git-shell.
+
+GIT_CVSSERVER_BASE_PATH takes the place of the argument to --base-path.
+
+GIT_CVSSERVER_ROOT specifies a single-directory whitelist. The
+repository must still be configured to allow access through
+git-cvsserver, as described above.
+
+When these environment variables are set, the corresponding
+command-line arguments may not be used.
+
 Eclipse CVS Client Notes
 ------------------------
 

git-cvsserver.perl

@@ -104,14 +104,16 @@
 my $usage =
     "Usage: git cvsserver [options] [pserver|server] [<directory> ...]\n".
     "    --base-path <path>  : Prepend to requested CVSROOT\n".
+    "                          Can be read from GIT_CVSSERVER_BASE_PATH\n".
     "    --strict-paths      : Don't allow recursing into subdirectories\n".
     "    --export-all        : Don't check for gitcvs.enabled in config\n".
     "    --version, -V       : Print version information and exit\n".
     "    --help, -h, -H      : Print usage information and exit\n".
     "\n".
     "<directory> ... is a list of allowed directories. If no directories\n".
     "are given, all are allowed. This is an additional restriction, gitcvs\n".
-    "access still needs to be enabled by the gitcvs.enabled config option.\n";
+    "access still needs to be enabled by the gitcvs.enabled config option.\n".
+    "Alternately, one directory may be specified in GIT_CVSSERVER_ROOT.\n";
 
 my @opts = ( 'help|h|H', 'version|V',
 	     'base-path=s', 'strict-paths', 'export-all' );
@@ -148,6 +150,24 @@
     die "--export-all can only be used together with an explicit whitelist\n";
 }
 
+# Environment handling for running under git-shell
+if (exists $ENV{GIT_CVSSERVER_BASE_PATH}) {
+    if ($state->{'base-path'}) {
+	die "Cannot specify base path both ways.\n";
+    }
+    my $base_path = $ENV{GIT_CVSSERVER_BASE_PATH};
+    $state->{'base-path'} = $base_path;
+    $log->debug("Picked up base path '$base_path' from environment.\n");
+}
+if (exists $ENV{GIT_CVSSERVER_ROOT}) {
+    if (@{$state->{allowed_roots}}) {
+	die "Cannot specify roots both ways: @ARGV\n";
+    }
+    my $allowed_root = $ENV{GIT_CVSSERVER_ROOT};
+    $state->{allowed_roots} = [ $allowed_root ];
+    $log->debug("Picked up allowed root '$allowed_root' from environment.\n");
+}
+
 # if we are called with a pserver argument,
 # deal with the authentication cat before entering the
 # main loop