img: stop ImageMagick trying to be clever if filenames contain a colon

$im->Read() takes a filename-like argument with several sets of special
syntax. Most of the possible metacharacters are escaped by the
default `wiki_file_chars` (and in any case not particularly disruptive),
but the colon ":" is not.

It seems the way to force ImageMagick to treat colons within the
filename as literal is to prepend a colon, so do that.

IkiWiki/Plugin/img.pm

@@ -76,7 +76,7 @@ sub preprocess (@) {
 	my $im = Image::Magick->new();
 	my $imglink;
 	my $imgdatalink;
-	my $r = $im->Read("$srcfile\[$pagenumber]");
+	my $r = $im->Read(":$srcfile\[$pagenumber]");
 	error sprintf(gettext("failed to read %s: %s"), $file, $r) if $r;
 
 	if (! defined $im->Get("width") || ! defined $im->Get("height")) {

debian/changelog

@@ -1,6 +1,7 @@
 ikiwiki (3.20150611) UNRELEASED; urgency=medium
 
   * inline: change default sort order from age to "age title" for determinism
+  * img: avoid ImageMagick misinterpreting filenames containing a colon
 
  -- Simon McVittie <smcv@debian.org>  Thu, 11 Jun 2015 08:32:35 +0100
 

doc/bugs/Colons___8216__:__8217___should_not_be_allowed_in_image_filenames.mdwn

@@ -9,4 +9,28 @@ However, `Image::Magick` doesn't seem to like page selection for filenames conta
     $ identify 'screenshot_2015-06-06_18-37-53.png[0]'
     screenshot_2015-06-06_18-37-53.png[0]=>screenshot_2015-06-06_18-37-53.png PNG 453x122 453x122+0+0 8-bit sRGB 11.2KB 0.000u 0:00.000
 
-This might be an imagemagick bug, but it's also possible that colons are interpreted somehow.  Anyway, to render such images properly in ikiwiki I had to remove the colons.  An easy fix is to remove ‘:’ from `wiki_file_chars`, but this can break existing installations.  A better solution would be to make `IkiWiki::Plugin::img` croak on such image filenames (which anyway are currently not rendered, but `Image::Magick`'s error message is quite cryptic).
+This might be an imagemagick bug, but it's also possible that colons
+are interpreted somehow.
+
+> Yes they are: ImageMagick has syntax to force the file to be
+> interpreted as a particular format, like gif:foobar.img to
+> read foobar.img as a GIF, or to use unusual I/O patterns, like
+> fd:5. --[[smcv]]
+
+Anyway, to render such images properly in ikiwiki I had to remove
+the colons.  An easy fix is to remove ‘:’ from `wiki_file_chars`,
+but this can break existing installations.
+
+> I think we should remove `:` from the default `wiki_file_chars`
+> either as soon as we have a way to handle backwards compatibility,
+> or in ikiwiki 4; but you're right that it's a compat problem,
+> so we can't just do that as a solution. --s
+
+A better solution would be to make `IkiWiki::Plugin::img` croak on
+such image filenames (which anyway are currently not rendered, but
+`Image::Magick`'s error message is quite cryptic).
+
+> Better still would be to fix the bug by escaping the filename
+> so ImageMagick treats it as just a filename. It seems the way
+> to do that is to call `Read(":hello:world.png")` instead of
+> `Read("hello:world.png")`, which I have now [[done]]. --s

t/img.t

@@ -24,6 +24,11 @@ my $SVGS_WORK = defined $magick->QueryFormat("svg");
 ok(! system("rm -rf t/tmp; mkdir -p t/tmp/in"));
 
 ok(! system("cp t/img/redsquare.png t/tmp/in/redsquare.png"));
+# colons in filenames are a corner case for img
+ok(! system("cp t/img/redsquare.png t/tmp/in/hello:world.png"));
+ok(! system("cp t/img/redsquare.png t/tmp/in/a:b:c.png"));
+ok(! system("cp t/img/redsquare.png t/tmp/in/a:b:c:d.png"));
+ok(! system("cp t/img/redsquare.png t/tmp/in/a:b:c:d:e:f:g:h:i:j.png"));
 
 if ($SVGS_WORK) {
 	writefile("emptysquare.svg", "t/tmp/in",
@@ -42,6 +47,9 @@ writefile("imgconversions.mdwn", "t/tmp/in", <<EOF
 [[!img redsquare.png]]
 [[!img redsquare.png size=10x]]
 [[!img redsquare.png size=30x50]] expecting 30x30
+[[!img hello:world.png size=x8]] expecting 8x8
+[[!img a:b:c.png size=x4]]
+[[!img a:b:c:d:e:f:g:h:i:j.png size=x6]]
 $maybe_svg_img
 [[!img twopages.pdf size=12x]]
 [[!img twopages.pdf size=16x pagenumber=1]]
@@ -57,7 +65,7 @@ ok(! system($command));
 sub size($) {
 	my $filename = shift;
 	my $im = Image::Magick->new();
-	my $r = $im->Read($filename);
+	my $r = $im->Read(":$filename");
 	return "no image" if $r;
 	my $w = $im->Get("width");
 	my $h = $im->Get("height");
@@ -78,6 +86,10 @@ if ($SVGS_WORK) {
 
 is(size("$outpath/12x-twopages.png"), "12x12");
 is(size("$outpath/16x-p1-twopages.png"), "16x2");
+ok($outhtml =~ /width="8" height="8".*expecting 8x8/);
+is(size("$outpath/x8-hello:world.png"), "8x8");
+is(size("$outpath/x4-a:b:c.png"), "4x4");
+is(size("$outpath/x6-a:b:c:d:e:f:g:h:i:j.png"), "6x6");
 
 # now let's remove them again
 
@@ -90,6 +102,9 @@ if (1) { # for easier testing
 	ok(! -e "$outpath/10x-simple-svg.png");
 	ok(! -e "$outpath/10x-simple-pdf.png");
 	ok(! -e "$outpath/10x-p1-simple-pdf.png");
+	ok(! -e "$outpath/x8-hello:world.png");
+	ok(! -e "$outpath/x4-a:b:c.png");
+	ok(! -e "$outpath/x6-a:b:c:d:e:f:g:h:i:j.png");
 
 	# cleanup
 	ok(! system("rm -rf t/tmp"));