config: cache results of kernel checks

Kernel checks are the heaviest part of the configure checks. This allows
the results to be cached through the normal autoconf cache.

Since we don't want to reuse cached values for different kernels, but
don't want to discard the entire cache on every kernel, we instead add a
short checksum to kernel config cache keys, based on the version and
path, so the cache can hold results for multiple different kernels.

Sponsored-by: https://despairlabs.com/sponsor/
Signed-off-by: Rob Norris <[email protected]>

Author: robn

config/kernel.m4

@@ -463,6 +463,11 @@ AC_DEFUN([ZFS_AC_KERNEL], [
 	AC_SUBST(LINUX)
 	AC_SUBST(LINUX_OBJ)
 	AC_SUBST(LINUX_VERSION)
+
+	dnl # create a relatively unique numeric checksym based on the kernel
+	dnl # version and path. this is included in the cache key below,
+	dnl # allowing different cached values for different kernels
+	_zfs_linux_cache_checksum=$(echo ${kernelsrc} {$kernelbuild} ${kernsrcver} | cksum | cut -f1 -d' ')
 ])
 
 AC_DEFUN([ZFS_AC_KERNEL_VERSION_WARNING], [
@@ -810,14 +815,18 @@ dnl # must never depend on the results of previous tests.  Each test
 dnl # needs to be entirely independent.
 dnl #
 AC_DEFUN([ZFS_LINUX_TEST_SRC], [
-	ZFS_LINUX_CONFTEST_C([ZFS_LINUX_TEST_PROGRAM([[$2]], [[$3]],
-	    [["Dual BSD/GPL"]])], [$1])
-	ZFS_LINUX_CONFTEST_MAKEFILE([$1], [yes], [$4])
-
-	AS_IF([ test -n "$5" ], [
-		ZFS_LINUX_CONFTEST_C([ZFS_LINUX_TEST_PROGRAM(
-		    [[$2]], [[$3]], [[$5]])], [$1_license])
-		ZFS_LINUX_CONFTEST_MAKEFILE([$1_license], [yes], [$4])
+	cachevar="zfs_cv_kernel_[$1]_$_zfs_linux_cache_checksum"
+	eval "cacheval=\$$cachevar"
+	AS_IF([test "x$cacheval" = "x"], [
+		ZFS_LINUX_CONFTEST_C([ZFS_LINUX_TEST_PROGRAM([[$2]], [[$3]],
+		    [["Dual BSD/GPL"]])], [$1])
+		ZFS_LINUX_CONFTEST_MAKEFILE([$1], [yes], [$4])
+
+		AS_IF([ test -n "$5" ], [
+			ZFS_LINUX_CONFTEST_C([ZFS_LINUX_TEST_PROGRAM(
+			    [[$2]], [[$3]], [[$5]])], [$1_license])
+			ZFS_LINUX_CONFTEST_MAKEFILE([$1_license], [yes], [$4])
+		])
 	])
 ])
 
@@ -829,14 +838,23 @@ dnl # $2 - run on success (valid .ko generated)
 dnl # $3 - run on failure (unable to compile)
 dnl #
 AC_DEFUN([ZFS_LINUX_TEST_RESULT], [
-	AS_IF([test -d build/$1], [
-		AS_IF([test -f build/$1/$1.ko], [$2], [$3])
-	], [
-		AC_MSG_ERROR([
+	cachevar="zfs_cv_kernel_[$1]_$_zfs_linux_cache_checksum"
+	AC_CACHE_VAL([$cachevar], [
+		AS_IF([test -d build/$1], [
+			AS_IF([test -f build/$1/$1.ko], [
+				eval "$cachevar=yes"
+			], [
+				eval "$cachevar=no"
+			])
+		], [
+			AC_MSG_ERROR([
 	*** No matching source for the "$1" test, check that
 	*** both the test source and result macros refer to the same name.
+			])
 		])
 	])
+	eval "cacheval=\$$cachevar"
+	AS_IF([test "x$cacheval" = "xyes"], [$2], [$3])
 ])
 
 dnl #
@@ -865,15 +883,24 @@ dnl # verify symbol exports, unless --enable-linux-builtin was provided to
 dnl # configure.
 dnl #
 AC_DEFUN([ZFS_LINUX_TEST_RESULT_SYMBOL], [
-	AS_IF([ ! test -f build/$1/$1.ko], [
-		$5
-	], [
-		AS_IF([test "x$enable_linux_builtin" != "xyes"], [
-			ZFS_CHECK_SYMBOL_EXPORT([$2], [$3], [$4], [$5])
+	cachevar="zfs_cv_kernel_[$1]_$_zfs_linux_cache_checksum"
+	AC_CACHE_VAL([$cachevar], [
+		AS_IF([ ! test -f build/$1/$1.ko], [
+			eval "$cachevar=no"
 		], [
-			$4
+			AS_IF([test "x$enable_linux_builtin" != "xyes"], [
+				ZFS_CHECK_SYMBOL_EXPORT([$2], [$3], [
+					eval "$cachevar=yes"
+				], [
+					eval "$cachevar=no"
+				])
+			], [
+				eval "$cacheval=yes"
+			])
 		])
 	])
+	eval "cacheval=\$$cachevar"
+	AS_IF([test "x$cacheval" = "xyes"], [$4], [$5])
 ])
 
 dnl #