gitweb: use highlight's shebang detection

The "highlight" binary can, in some cases, determine the language type
by the means of file contents, for example the shebang in the first line
for some scripting languages.  Make use of this autodetection for files
which syntax is not known by gitweb.  In that case, pass the blob
contents to "highlight --force"; the parameter is needed to make it
always generate HTML output (which includes HTML-escaping).

Although we now run highlight on files which do not end up highlighted,
performance is virtually unaffected because when we call highlight, it
is used for escaping HTML.  In the case that highlight is used, gitweb
calls sanitize() instead of esc_html(), and the latter is significantly
slower (it does more, being roughly a superset of sanitize()).  Simple
benchmark comparing performance of 'blob' view of files without syntax
highlighting in gitweb before and after this change indicates ±1%
difference in request time for all file types.  Benchmark was performed
on local instance on Debian, using Apache/2.4.23 web server and CGI.

Document the feature and improve syntax highlight documentation, add
test to ensure gitweb doesn't crash when language detection is used.

Signed-off-by: Ian Kelling <ian@iankelling.org>
Acked-by: Jakub Narębski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 14 insertions, 7 deletions

diff --git a/Documentation/gitweb.conf.txt b/Documentation/gitweb.conf.txt
index a79e350246..e6320891b1 100644
--- a/Documentation/gitweb.conf.txt
+++ b/Documentation/gitweb.conf.txt
@@ -246,13 +246,20 @@ $highlight_bin::
 	Note that 'highlight' feature must be set for gitweb to actually
 	use syntax highlighting.
 +
-*NOTE*: if you want to add support for new file type (supported by
-"highlight" but not used by gitweb), you need to modify `%highlight_ext`
-or `%highlight_basename`, depending on whether you detect type of file
-based on extension (for example "sh") or on its basename (for example
-"Makefile").  The keys of these hashes are extension and basename,
-respectively, and value for given key is name of syntax to be passed via
-`--syntax <syntax>` to highlighter.
+*NOTE*: for a file to be highlighted, its syntax type must be detected
+and that syntax must be supported by "highlight".  The default syntax
+detection is minimal, and there are many supported syntax types with no
+detection by default.  There are three options for adding syntax
+detection.  The first and second priority are `%highlight_basename` and
+`%highlight_ext`, which detect based on basename (the full filename, for
+example "Makefile") and extension (for example "sh").  The keys of these
+hashes are the basename and extension, respectively, and the value for a
+given key is the name of the syntax to be passed via `--syntax <syntax>`
+to "highlight".  The last priority is the "highlight" configuration of
+`Shebang` regular expressions to detect the language based on the first
+line in the file, (for example, matching the line "#!/bin/bash").  See
+the highlight documentation and the default config at
+/etc/highlight/filetypes.conf for more details.
 +
 For example if repositories you are hosting use "phtml" extension for
 PHP files, and you want to have correct syntax-highlighting for those