checksrc: add COPYRIGHTYEAR check

Forgetting to bump the year in the copyright clause when hacking has
been quite common among curl developers, but a traditional checksrc
check isn't a good fit as it would penalize anyone hacking on January
1st (among other things). This adds a more selective COPYRIGHTYEAR
check which intends to only cover the currently hacked on changeset.

The check for updated copyright year is currently not enforced on all
files but only on files edited and/or committed locally. This is due to
the amount of files which aren't updated with their correct copyright
year at the time of their respective commit.

To further avoid running this expensive check for every developer, it
adds a new local override mode for checksrc where a .checksrc file can
be used to turn on extended warnings locally.

Closes #3303
Reviewed-by: Daniel Stenberg <daniel@haxx.se>

1 files changed, 13 insertions, 0 deletions

diff --git a/docs/CHECKSRC.md b/docs/CHECKSRC.md
index 543ea5620..10e2f4d73 100644
--- a/docs/CHECKSRC.md
+++ b/docs/CHECKSRC.md
@@ -103,6 +103,19 @@ warnings are:
 - `UNUSEDIGNORE`: a checksrc inlined warning ignore was asked for but not used,
    that's an ignore that should be removed or changed to get used.
 
+### Extended warnings
+
+Some warnings are quite computationally expensive to perform, so they are
+turned off by default. To enable these warnings, place a `.checksrc` file in
+the directory where they should be activated with commands to enable the
+warnings you are interested in. The format of the file is to enable one
+warning per line like so: `enable <EXTENDEDWARNING>`
+
+Currently there is one extended warning which can be enabled:
+
+- `COPYRIGHTYEAR`: the current changeset hasn't updated the copyright year in
+   the source file
+
 ## Ignore certain warnings
 
 Due to the nature of the source code and the flaws of the checksrc tool, there