Bug 1415971 - Inline sccache.mk into client.mk; r=nalexander

sccache.mk is the only thing that uses MOZ_PREFLIGHT_ALL and
MOZ_POSTFLIGHT_ALL. We're trying to kill client.mk and these
variables are next on the chopping block.

This commit essentially moves the logic of sccache.mk inline into
client.mk. Initially, I wanted to move the management of the
sccache daemon to Python as part of what `mach build` invokes.
However, the sccache daemon needs access to the make jobserver.
Since we don't have an active make process in `mach`, this is
obviously problematic. The sccache daemon is also used by
configure, which is launched from client.mk. So the best we
can do right now is move the sccache daemon logic into client.mk.

As part of the port, we pass the path to the sccache binary via
an environment variable. This feels slightly better than hardcoding
the path that automation uses.

MozReview-Commit-ID: zcOYR4I1OG

--HG--
extra : rebase_source : 305a237dc9f5bd96e4aa83d491ac2498fe3c3ba0

build/mozconfig.cache

@@ -119,8 +119,7 @@ else
     esac
     export CCACHE="$topsrcdir/sccache2/sccache${suffix}"
     export SCCACHE_VERBOSE_STATS=1
-    mk_add_options MOZ_PREFLIGHT_ALL+=build/sccache.mk
-    mk_add_options MOZ_POSTFLIGHT_ALL+=build/sccache.mk
+    mk_add_options MOZBUILD_MANAGE_SCCACHE_DAEMON=${topsrcdir}/sccache2/sccache
     mk_add_options "UPLOAD_EXTRA_FILES+=sccache.log.gz"
     case "$platform" in
     win*)

client.mk

@@ -164,6 +164,18 @@ build preflight_all::
 	done
 endif
 
+# In automation, manage an sccache daemon. The starting of the server
+# needs to be in a make file so sccache inherits the jobserver.
+ifdef MOZBUILD_MANAGE_SCCACHE_DAEMON
+build::
+	# Terminate any sccache server that might still be around.
+	-$(MOZBUILD_MANAGE_SCCACHE_DAEMON) --stop-server > /dev/null 2>&1
+	# 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 --%, $(MAKEFLAGS))),,+)env RUST_LOG=sccache::compiler=debug SCCACHE_ERROR_LOG=$(OBJDIR)/dist/sccache.log $(MOZBUILD_MANAGE_SCCACHE_DAEMON) --start-server
+endif
+
 ####################################
 # Configure
 
@@ -288,6 +300,12 @@ build::
 	$(MAKE) -f $(TOPSRCDIR)/client.mk automation/build
 endif
 
+ifdef MOZBUILD_MANAGE_SCCACHE_DAEMON
+build::
+	# Terminate sccache server. This prints sccache stats.
+	-$(MOZBUILD_MANAGE_SCCACHE_DAEMON) --stop-server
+endif
+
 ifdef MOZ_POSTFLIGHT_ALL
 build postflight_all::
 	set -e; \