Bug 1380964 - Get and upload useful sccache logs. r=gps

When investigating why a particular build got a particularly high number
of cache misses, it is useful to know exactly *what* had cache misses,
and you can't get the missing information after the fact.

We used to have some stats stored in a log file uploaded as artifact
with the old python sccache per bug 1005495 (and before that inline in
the build log), and we actually still have remains of that in the form
of a EXTRA_UPLOAD_FILES in build/mozconfig.cache.

Let's reuse that, and get some useful logs from sccache. Ideally sccache
would give better logs, but those are sufficient for now. We'll figure
things out eventually. mozilla/sccache#151

--HG--
extra : rebase_source : a2e7093697a5b720ab28f155560211d6c0a4347e

Makefile.in

@@ -248,6 +248,9 @@ default::
 	@echo "===SCCACHE STATS==="
 	-$(CCACHE) --show-stats
 	@echo "==================="
+# Ideally we'd do that in the same file as we set the sccache.log location for
+# sccache, but it's too late in the build.
+	-gzip -9 $(DIST)/sccache.log
 endif
 
 distclean::

build/sccache.mk

@@ -15,7 +15,7 @@ preflight_all:
 	# Start a new server, ensuring it gets the jobserver file descriptors
 	# from make (but don't use the + prefix when make -n is used, so that
 	# the command doesn't run in that case)
-	$(if $(findstring n,$(filter-out --%, $(MAKEFLAGS))),,+)$(TOPSRCDIR)/sccache2/sccache --start-server
+	$(if $(findstring n,$(filter-out --%, $(MAKEFLAGS))),,+)env RUST_LOG=sccache::compiler=debug SCCACHE_ERROR_LOG=$(OBJDIR)/dist/sccache.log $(TOPSRCDIR)/sccache2/sccache --start-server
 
 postflight_all:
 	# Terminate sccache server. This prints sccache stats.