Merge branch 'rs/qsort-s'

A few codepaths had to rely on a global variable when sorting
elements of an array because sort(3) API does not allow extra data
to be passed to the comparison function.  Use qsort_s() when
natively available, and a fallback implementation of it when not,
to eliminate the need, which is a prerequisite for making the
codepath reentrant.

* rs/qsort-s:
  ref-filter: use QSORT_S in ref_array_sort()
  string-list: use QSORT_S in string_list_sort()
  perf: add basic sort performance test
  add QSORT_S
  compat: add qsort_s()

7 files changed, 146 insertions, 12 deletions

diff --git a/Makefile b/Makefile
index 27afd0f37..53ecc84e2 100644
--- a/Makefile
+++ b/Makefile
@@ -279,6 +279,9 @@ all::
 # is a simplified version of the merge sort used in glibc. This is
 # recommended if Git triggers O(n^2) behavior in your platform's qsort().
 #
+# Define HAVE_ISO_QSORT_S if your platform provides a qsort_s() that's
+# compatible with the one described in C11 Annex K.
+#
 # Define UNRELIABLE_FSTAT if your system's fstat does not return the same
 # information on a not yet closed file that lstat would return for the same
 # file after it was closed.
@@ -1418,6 +1421,11 @@ ifdef INTERNAL_QSORT
 	COMPAT_CFLAGS += -DINTERNAL_QSORT
 	COMPAT_OBJS += compat/qsort.o
 endif
+ifdef HAVE_ISO_QSORT_S
+	COMPAT_CFLAGS += -DHAVE_ISO_QSORT_S
+else
+	COMPAT_OBJS += compat/qsort_s.o
+endif
 ifdef RUNTIME_PREFIX
 	COMPAT_CFLAGS += -DRUNTIME_PREFIX
 endif
diff --git a/compat/qsort_s.c b/compat/qsort_s.c
new file mode 100644
index 000000000..52d1f0a73
--- /dev/null
+++ b/compat/qsort_s.c
@@ -0,0 +1,69 @@
+#include "../git-compat-util.h"
+
+/*
+ * A merge sort implementation, simplified from the qsort implementation
+ * by Mike Haertel, which is a part of the GNU C Library.
+ * Added context pointer, safety checks and return value.
+ */
+
+static void msort_with_tmp(void *b, size_t n, size_t s,
+			   int (*cmp)(const void *, const void *, void *),
+			   char *t, void *ctx)
+{
+	char *tmp;
+	char *b1, *b2;
+	size_t n1, n2;
+
+	if (n <= 1)
+		return;
+
+	n1 = n / 2;
+	n2 = n - n1;
+	b1 = b;
+	b2 = (char *)b + (n1 * s);
+
+	msort_with_tmp(b1, n1, s, cmp, t, ctx);
+	msort_with_tmp(b2, n2, s, cmp, t, ctx);
+
+	tmp = t;
+
+	while (n1 > 0 && n2 > 0) {
+		if (cmp(b1, b2, ctx) <= 0) {
+			memcpy(tmp, b1, s);
+			tmp += s;
+			b1 += s;
+			--n1;
+		} else {
+			memcpy(tmp, b2, s);
+			tmp += s;
+			b2 += s;
+			--n2;
+		}
+	}
+	if (n1 > 0)
+		memcpy(tmp, b1, n1 * s);
+	memcpy(b, t, (n - n2) * s);
+}
+
+int git_qsort_s(void *b, size_t n, size_t s,
+		int (*cmp)(const void *, const void *, void *), void *ctx)
+{
+	const size_t size = st_mult(n, s);
+	char buf[1024];
+
+	if (!n)
+		return 0;
+	if (!b || !cmp)
+		return -1;
+
+	if (size < sizeof(buf)) {
+		/* The temporary array fits on the small on-stack buffer. */
+		msort_with_tmp(b, n, s, cmp, buf, ctx);
+	} else {
+		/* It's somewhat large, so malloc it.  */
+		char *tmp = xmalloc(size);
+		msort_with_tmp(b, n, s, cmp, tmp, ctx);
+		free(tmp);
+	}
+	return 0;
+}
diff --git a/git-compat-util.h b/git-compat-util.h
index 87237b092..f46d40e4a 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -988,6 +988,17 @@ static inline void sane_qsort(void *base, size_t nmemb, size_t size,
 		qsort(base, nmemb, size, compar);
 }
 
+#ifndef HAVE_ISO_QSORT_S
+int git_qsort_s(void *base, size_t nmemb, size_t size,
+		int (*compar)(const void *, const void *, void *), void *ctx);
+#define qsort_s git_qsort_s
+#endif
+
+#define QSORT_S(base, n, compar, ctx) do {			\
+	if (qsort_s((base), (n), sizeof(*(base)), compar, ctx))	\
+		die("BUG: qsort_s() failed");			\
+} while (0)
+
 #ifndef REG_STARTEND
 #error "Git requires REG_STARTEND support. Compile with NO_REGEX=NeedsStartEnd"
 #endif
diff --git a/ref-filter.c b/ref-filter.c
index 5f4b08792..3820b21cc 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -1594,8 +1594,7 @@ static int cmp_ref_sorting(struct ref_sorting *s, struct ref_array_item *a, stru
 	return (s->reverse) ? -cmp : cmp;
 }
 
-static struct ref_sorting *ref_sorting;
-static int compare_refs(const void *a_, const void *b_)
+static int compare_refs(const void *a_, const void *b_, void *ref_sorting)
 {
 	struct ref_array_item *a = *((struct ref_array_item **)a_);
 	struct ref_array_item *b = *((struct ref_array_item **)b_);
@@ -1611,8 +1610,7 @@ static int compare_refs(const void *a_, const void *b_)
 
 void ref_array_sort(struct ref_sorting *sorting, struct ref_array *array)
 {
-	ref_sorting = sorting;
-	QSORT(array->items, array->nr, compare_refs);
+	QSORT_S(array->items, array->nr, compare_refs, sorting);
 }
 
 static void append_literal(const char *cp, const char *ep, struct ref_formatting_state *state)
diff --git a/string-list.c b/string-list.c
index 8c83cac18..45016ad86 100644
--- a/string-list.c
+++ b/string-list.c
@@ -211,21 +211,18 @@ struct string_list_item *string_list_append(struct string_list *list,
 			list->strdup_strings ? xstrdup(string) : (char *)string);
 }
 
-/* Yuck */
-static compare_strings_fn compare_for_qsort;
-
-/* Only call this from inside string_list_sort! */
-static int cmp_items(const void *a, const void *b)
+static int cmp_items(const void *a, const void *b, void *ctx)
 {
+	compare_strings_fn cmp = ctx;
 	const struct string_list_item *one = a;
 	const struct string_list_item *two = b;
-	return compare_for_qsort(one->string, two->string);
+	return cmp(one->string, two->string);
 }
 
 void string_list_sort(struct string_list *list)
 {
-	compare_for_qsort = list->cmp ? list->cmp : strcmp;
-	QSORT(list->items, list->nr, cmp_items);
+	QSORT_S(list->items, list->nr, cmp_items,
+		list->cmp ? list->cmp : strcmp);
 }
 
 struct string_list_item *unsorted_string_list_lookup(struct string_list *list,
diff --git a/t/helper/test-string-list.c b/t/helper/test-string-list.c
index 4a68967bd..c502fa16d 100644
--- a/t/helper/test-string-list.c
+++ b/t/helper/test-string-list.c
@@ -97,6 +97,31 @@ int cmd_main(int argc, const char **argv)
 		return 0;
 	}
 
+	if (argc == 2 && !strcmp(argv[1], "sort")) {
+		struct string_list list = STRING_LIST_INIT_NODUP;
+		struct strbuf sb = STRBUF_INIT;
+		struct string_list_item *item;
+
+		strbuf_read(&sb, 0, 0);
+
+		/*
+		 * Split by newline, but don't create a string_list item
+		 * for the empty string after the last separator.
+		 */
+		if (sb.buf[sb.len - 1] == '\n')
+			strbuf_setlen(&sb, sb.len - 1);
+		string_list_split_in_place(&list, sb.buf, '\n', -1);
+
+		string_list_sort(&list);
+
+		for_each_string_list_item(item, &list)
+			puts(item->string);
+
+		string_list_clear(&list, 0);
+		strbuf_release(&sb);
+		return 0;
+	}
+
 	fprintf(stderr, "%s: unknown function name: %s\n", argv[0],
 		argv[1] ? argv[1] : "(there was none)");
 	return 1;
diff --git a/t/perf/p0071-sort.sh b/t/perf/p0071-sort.sh
new file mode 100755
index 000000000..7c9a35a64
--- /dev/null
+++ b/t/perf/p0071-sort.sh
@@ -0,0 +1,26 @@
+#!/bin/sh
+
+test_description='Basic sort performance tests'
+. ./perf-lib.sh
+
+test_perf_default_repo
+
+test_expect_success 'setup' '
+	git ls-files --stage "*.[ch]" "*.sh" |
+	cut -f2 -d" " |
+	git cat-file --batch >unsorted
+'
+
+test_perf 'sort(1)' '
+	sort <unsorted >expect
+'
+
+test_perf 'string_list_sort()' '
+	test-string-list sort <unsorted >actual
+'
+
+test_expect_success 'string_list_sort() sorts like sort(1)' '
+	test_cmp_bin expect actual
+'
+
+test_done