checkpatch: warn when Reported-by: is not followed by Link:

Encourage patch authors to link to reports by issuing a warning, if a
Reported-by: is not accompanied by a link to the report.  Those links are
often extremely useful for any code archaeologist that wants to know more
about the backstory of a change than the commit message provides.  That
includes maintainers higher up in the patch-flow hierarchy, which is why
Linus asks developers to add such links [1, 2, 3].  To quote [1]:

> Again, the commit has a link to the patch *submission*, which is
> almost entirely useless. There's no link to the actual problem the
> patch fixes.
>
> [...]
>
> Put another way: I can see that
>
> Reported-by: Zhangfei Gao <[email protected]>
>
> in the commit, but I don't have a clue what the actual report was, and
> there really isn't enough information in the commit itself, except for
> a fairly handwavy "Device drivers might, for instance, still need to
> flush operations.."
>
> I don't want to know what device drivers _might_ do. I would want to
> have an actual pointer to what they do and where.

Another reason why these links are wanted: the ongoing regression tracking
efforts can only scale with them, as they allow the regression tracking
bot 'regzbot' to automatically connect tracked reports with patches that
are posted or committed to fix tracked regressions.

Link: https://lore.kernel.org/all/CAHk-=wjMmSZzMJ3Xnskdg4+GGz=5p5p+GSYyFBTh0f-DgvdBWg@mail.gmail.com/ [1]
Link: https://lore.kernel.org/all/CAHk-=wgs38ZrfPvy=nOwVkVzjpM3VFU1zobP37Fwd_h9iAD5JQ@mail.gmail.com/ [2]
Link: https://lore.kernel.org/all/CAHk-=wjxzafG-=J8oT30s7upn4RhBs6TX-uVFZ5rME+L5_DoJA@mail.gmail.com/ [3]
Link: https://lkml.kernel.org/r/bb5dfd55ea2026303ab2296f4a6df3da7dd64006.1674217480.git.linux@leemhuis.info
Signed-off-by: Kai Wasserbäch <[email protected]>
Co-developed-by: Thorsten Leemhuis <[email protected]>
Signed-off-by: Thorsten Leemhuis <[email protected]>
Cc: Andy Whitcroft <[email protected]>
Cc: Dwaipayan Ray <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Lukas Bulwahn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

scripts/checkpatch.pl

@@ -3157,8 +3157,20 @@ sub process {
 					     "Co-developed-by and Signed-off-by: name/email do not match \n" . "$here\n" . $rawline . "\n" .$rawlines[$linenr]);
 				}
 			}
+
+# check if Reported-by: is followed by a Link:
+			if ($sign_off =~ /^reported(?:|-and-tested)-by:$/i) {
+				if (!defined $lines[$linenr]) {
+					WARN("BAD_REPORTED_BY_LINK",
+					     "Reported-by: should be immediately followed by Link: to the report\n" . $herecurr . $rawlines[$linenr] . "\n");
+				} elsif ($rawlines[$linenr] !~ m{^link:\s*https?://}i) {
+					WARN("BAD_REPORTED_BY_LINK",
+					     "Reported-by: should be immediately followed by Link: with a URL to the report\n" . $herecurr . $rawlines[$linenr] . "\n");
+				}
+			}
 		}
 
+
 # Check Fixes: styles is correct
 		if (!$in_header_lines &&
 		    $line =~ /^\s*fixes:?\s*(?:commit\s*)?[0-9a-f]{5,}\b/i) {