headerscheck ccache support

Currently, headerscheck and cpluspluscheck are very slow, and they
defeat use of ccache.  This fixes that, and now they are much faster.

The problem was that the test files are created in a randomly-named
directory (`mktemp -d /tmp/$me.XXXXXX`), and this directory is
mentioned on the compiler command line, which is part of the cache
key.

The solution is to create the test files in the build directory.  For
example, for src/include/storage/ipc.h, we generate

    tmp_headerscheck_c/src_include_storage_ipc_h.c (or .cpp)

Now ccache works.  (And it's also a bit easier to debug everything
with this naming.)

(The subdirectory is used to keep the cleanup trap simple.)

The observed speedup on Cirrus CI for headerscheck plus cpluspluscheck
is from about 1min 20s to only 20s.  In local use, the speedups are
similar.

Co-authored-by: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/b49e74d4-3cf9-4d1c-9dce-09f75e55d026%40eisentraut.org

.cirrus.tasks.yml

@@ -1005,16 +1005,15 @@ task:
   ###
   # Verify headerscheck / cpluspluscheck succeed
   #
-  # - Don't use ccache, the files are uncacheable, polluting ccache's
-  #   cache
   # - Use -fmax-errors, as particularly cpluspluscheck can be very verbose
   ###
   always:
     headers_headerscheck_script: |
       time ./configure \
         ${LINUX_CONFIGURE_FEATURES} \
+        --cache gcc.cache \
         --quiet \
-        CC="gcc" CXX"=g++" CLANG="clang"
+        CC="ccache gcc" CXX="ccache g++" CLANG="ccache clang"
       make -s -j${BUILD_JOBS} clean
       time make -s headerscheck EXTRAFLAGS='-fmax-errors=10'
     headers_cpluspluscheck_script: |

src/tools/pginclude/headerscheck

@@ -73,8 +73,10 @@ else
 	COMPILER_FLAGS="$CPPFLAGS $CFLAGS $ICU_CFLAGS $LLVM_CPPFLAGS"
 fi
 
-# Create temp directory.
-tmp=`mktemp -d /tmp/$me.XXXXXX`
+# Create temp directory.  Help ccache by using a stable name.  Include
+# extension to allow running C and C++ checks in parallel.
+tmp="tmp_${me}_${ext}"
+mkdir -p "$tmp"
 
 trap "ret=$?; rm -rf $tmp; exit $ret" 0 1 2 3 15
 
@@ -200,6 +202,9 @@ do
 		test "$f" = src/bin/pg_dump/pg_dump.h && continue
 	fi
 
+	# Help ccache by using a stable name.  Remove slashes and dots.
+	test_file_name=$(printf '%s' "$f" | tr '/.' '__')
+
 	# OK, create .c file to include this .h file.
 	{
 	    $cplusplus && echo 'extern "C" {'
@@ -232,7 +237,7 @@ do
 	    esac
 	    echo "#include \"$f\""
 	    $cplusplus && echo '};'
-	} >$tmp/test.$ext
+	} >$tmp/$test_file_name.$ext
 
 	# Some subdirectories need extra -I switches.
 	case "$f" in
@@ -254,10 +259,12 @@ do
 	if ! $COMPILER $COMPILER_FLAGS -I $builddir -I $srcdir \
 		-I $builddir/src/include -I $srcdir/src/include \
 		-I $builddir/src/interfaces/libpq -I $srcdir/src/interfaces/libpq \
-		$EXTRAINCLUDES $EXTRAFLAGS -c $tmp/test.$ext -o $tmp/test.o
+		$EXTRAINCLUDES $EXTRAFLAGS -c "$tmp/$test_file_name.$ext" -o "$tmp/$test_file_name.o"
 	then
 		exit_status=1
 	fi
+
+	rm -f "$tmp/$test_file_name.$ext" "$tmp/$test_file_name.o"
 done
 
 exit $exit_status