loader: add local-cgi

Add w3m-style local CGI support.

It is not quite as powerful as w3m's local CGI, because it lacks an
equivalent to W3m-control. Not sure if it's worth adding; we certainly
shouldn't allow passing JS in headers, but a custom language for
headers does not sound like a great idea either...

eh, idk. also, TODO add multipart

3 files changed, 143 insertions, 3 deletions

diff --git a/doc/cha.1 b/doc/cha.1
index e2b7d35d..aad18ed4 100644
--- a/doc/cha.1
+++ b/doc/cha.1
@@ -123,4 +123,5 @@ configuration option is not set.
 Configuration options are described in \fBcha-config\fR(5).
 
 .SH SEE ALSO
-\fBcha-mailcap\fR(5), \fBcha-mime.types\fR(5)
+\fBcha-mailcap\fR(5), \fBcha-mime.types\fR(5), \fBcha-config\fR(5),
+\fBcha-localcgi\fR(5)
diff --git a/doc/config.md b/doc/config.md
index ad1807c5..6c933cdc 100644
--- a/doc/config.md
+++ b/doc/config.md
@@ -171,13 +171,41 @@ the line number.</td>
 <tr>
 <td>mailcap</td>
 <td>array of paths</td>
-<td>Search path for [mailcap](mailcap.md) files.</td>
+<td>Search path for
+<!-- MANOFF -->
+[mailcap](mailcap.md) files.
+<!-- MANON -->
+<!-- MANON
+mailcap (**cha-mailcap**(5))
+MANOFF -->
+files.</td>
 </tr>
 
 <tr>
 <td>mime-types</td>
 <td>array of paths</td>
-<td>Search path for [mime.types](mime.types.md) files.</td>
+<td>Search path for
+<!-- MANOFF -->
+[mime.types](mime.types.md)
+<!-- MANON -->
+<!-- MANON
+mime.types (**cha-mime.types**(5))
+MANOFF -->
+files.</td>
+</tr>
+
+<tr>
+<td>cgi-dir</td>
+<td>array of paths</td>
+<td>Search path for
+<!-- MANOFF -->
+[mime.types](mime.types.md)
+[local CGI](localcgi.md)
+<!-- MANON -->
+<!-- MANON
+local CGI (**cha-localcgi**(5))
+MANOFF -->
+scripts.</td>
 </tr>
 
 </table>
diff --git a/doc/localcgi.md b/doc/localcgi.md
new file mode 100644
index 00000000..ba7d5232
--- /dev/null
+++ b/doc/localcgi.md
@@ -0,0 +1,111 @@
+<!-- MANON
+% cha-localcgi(5) | Local CGI support in Chawan
+MANOFF -->
+
+# Local CGI support in Chawan
+
+Chawan supports the invocation of CGI scripts locally. This feature can be
+used in the following way:
+
+* All local CGI scripts must be placed in a directory specified in
+  `external.cgi-dir`. Multiple directories can be specified in an array too,
+  and directories specified first have higher precedence.
+* Then, a CGI script in one of these directories can be executed by visiting
+  the URL `cgi-bin:script-name`. $PATH_INFO and $QUERY_STRING are set as
+  normal, i.e. `cgi-bin:script-name/abcd?defgh=ijkl` will set $PATH_INFO to
+  `/abcd`, and $QUERY_STRING to `defgh=ijkl`.
+
+Further notes on processing CGI paths:
+
+* The URL must be opaque, so you must not add a double slash after the scheme.
+  e.g. `cgi-bin://script-name` will NOT work, only `cgi-bin:script-name`.
+* Absolute paths are accepted as e.g. `cgi-bin:/path/to/cgi/dir/script-name`.
+  Note however, that this only works if `/path/to/cgi/dir` has already been
+  specified as a CGI directory in `external.cgi-dir`.
+
+Note that this is different from w3m's cgi-bin functionality, in that we
+use a custom scheme for local CGI instead of interpreting all requests to
+a designated path as a CGI request. Also, for now Chawan has no equivalent
+to the W3m-control headers (but this may change in the future).
+
+## Environment variables
+
+Chawan sets the following environment variables:
+
+* `SERVER_SOFTWARE="Chawan"`
+* `SERVER_PROTOCOL="HTTP/1.0"`
+* `SERVER_NAME="localhost"`
+* `SERVER_PORT="80"`
+* `REMOTE_HOST="localhost"`
+* `REMOTE_ADDR="127.0.0.1"`
+* `GATEWAY_INTERFACE="CGI/1.1"`
+* `SCRIPT_NAME="/cgi-bin/script-name"` if called with a relative path, and
+  `"/path/to/script/script-name"` if called with an absolute path.
+* `SCRIPT_FILENAME="/path/to/script/script-name"`
+* `QUERY_STRING=` the query string (i.e. `URL.search`). Note that this
+  variable is percent-encoded.
+* `PATH_INFO=` everything after the script's path name,
+  e.g. for `cgi-bin:script-name/abcd/efgh` `"/abcd/efgh"`. Note that this
+  variable is NOT percent-encoded.
+* `REQUEST_URI="$SCRIPT_NAME/$PATH_INFO?$QUERY_STRING`
+* `REQUEST_METHOD=` HTTP method used for making the request, e.g. GET or POST
+* `CONTENT_TYPE=` for POST requests, the Content-Type header. Not set for
+  other request types (e.g. GET).
+* `CONTENT_LENGTH=` the content length, if $CONTENT_TYPE has been set.
+* `HTTP_PROXY=` and (lower case) `http_proxy=`: the proxy URL if a proxy
+  has been set and its scheme is either `http` or `https`.
+* `ALL_PROXY=` if a proxy has been set, the proxy URL.
+* `HTTP_COOKIE=` if set, the Cookie header.
+* `HTTP_REFERER=` if set, the Referer header.
+
+## Request body
+
+If the request body is not empty, it is streamed into the program through
+the standard input.
+
+NOTE: multipart requests are not implemented yet. This will be fixed in
+the future.
+
+## Troubleshooting
+
+Note that standard error is redirected to the browser console (by default,
+M-cM-c). This makes it easy to debug a misbehaving CGI script, but may also
+slow down the browser in case of excessive logging. If this is not the
+desired behavior, we recommend wrapping your script into a shell script that
+redirects stderr to /dev/null.
+
+### My script is returning a "no local-CGI directory configured" error message.
+
+Configure a local-CGI directory using `external.cgi-dir`.
+
+e.g. you could add this to your config.toml:
+
+```toml
+[external]
+cgi-dir = "/usr/local/libexec/chawan/cgi-bin"
+```
+
+and then put your script in `/usr/local/libexec/chawan/cgi-bin`.
+
+### My script is returning an "invalid CGI path" error message.
+
+Make sure that you did not include leading slashes. Reminder:
+`cgi-bin://script-name` does not work, use `cgi-bin:script-name`.
+
+### My script is returning a "CGI file not found" error message.
+
+Double check that your CGI script is in the correct location. Also, make
+sure that you are not accidentally calling the script with an absolute path via
+`cgi-bin:/script-name` (instead of the correct `cgi-bin:script-name`).
+
+Also, make sure `external.cgi-dir` is set to the directory your script is in.
+
+### My script returns a page saying "Failed to execute script".
+
+This means the `execl` call to the script failed. Make sure that your CGI
+script's executable bit is set, i.e. run `chmod +x /path/to/cgi/script`.
+
+### My script is returning a "failed to set up CGI script" error message.
+
+This means that either `pipe` or `fork` failed. Something strange is going on
+with your system; we recommend exorcism. (Maybe you are running out of memory?)