Win32: keep the environment sorted

The Windows environment is sorted, keep it that way for O(log n)
environment access.

Change compareenv to compare only the keys, so that it can be used to
find an entry irrespective of the value.

Change lookupenv to binary seach for an entry. Return one's complement of
the insert position if not found (libc's bsearch returns NULL).

Replace MSVCRT's getenv with a minimal do_getenv based on the binary search
function.

Change do_putenv to insert new entries at the correct position. Simplify
the function by swapping if conditions and using memmove instead of for
loops.

Move qsort from make_environment_block to mingw_startup. We still need to
sort on startup to make sure that the environment is sorted according to
our compareenv function (while Win32 / CreateProcess requires the
environment block to be sorted case-insensitively, CreateProcess currently
doesn't enforce this, and some applications such as bash just don't care).

Note that environment functions are _not_ thread-safe and are not required
to be so by POSIX, the application is responsible for synchronizing access
to the environment. MSVCRT's getenv and our new getenv implementation are
better than that in that they are thread-safe with respect to other getenv
calls as long as the environment is not modified. Git's indiscriminate use
of getenv in background threads currently requires this property.

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Stepan Kasal <kasal@ucw.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -906,13 +906,6 @@ static int environ_size = 0;
 /* allocated size of environ array, in bytes */
 static int environ_alloc = 0;
 
-static int compareenv(const void *a, const void *b)
-{
-	char *const *ea = a;
-	char *const *eb = b;
-	return strcasecmp(*ea, *eb);
-}
-
 /*
  * Create environment block suitable for CreateProcess. Merges current
  * process environment and the supplied environment changes.
@@ -934,9 +927,6 @@ static wchar_t *make_environment_block(char **deltaenv)
 	for (i = 0; deltaenv && deltaenv[i]; i++)
 		size = do_putenv(tmpenv, deltaenv[i], size, 0);
 
-	/* environment must be sorted */
-	qsort(tmpenv, size - 1, sizeof(char*), compareenv);
-
 	/* create environment block from temporary environment */
 	for (i = 0; tmpenv[i]; i++) {
 		size = 2 * strlen(tmpenv[i]) + 2; /* +2 for final \0 */
@@ -1194,16 +1184,42 @@ int mingw_kill(pid_t pid, int sig)
 	return -1;
 }
 
-static int lookupenv(char **env, const char *name, size_t nmln)
-{
-	int i;
+/*
+ * Compare environment entries by key (i.e. stopping at '=' or '\0').
+ */
+static int compareenv(const void *v1, const void *v2)
+{
+	const char *e1 = *(const char**)v1;
+	const char *e2 = *(const char**)v2;
+
+	for (;;) {
+		int c1 = *e1++;
+		int c2 = *e2++;
+		c1 = (c1 == '=') ? 0 : tolower(c1);
+		c2 = (c2 == '=') ? 0 : tolower(c2);
+		if (c1 > c2)
+			return 1;
+		if (c1 < c2)
+			return -1;
+		if (c1 == 0)
+			return 0;
+	}
+}
 
-	for (i = 0; env[i]; i++) {
-		if (!strncasecmp(env[i], name, nmln) && '=' == env[i][nmln])
-			/* matches */
-			return i;
+static int bsearchenv(char **env, const char *name, size_t size)
+{
+	unsigned low = 0, high = size;
+	while (low < high) {
+		unsigned mid = low + ((high - low) >> 1);
+		int cmp = compareenv(&env[mid], &name);
+		if (cmp < 0)
+			low = mid + 1;
+		else if (cmp > 0)
+			high = mid;
+		else
+			return mid;
 	}
-	return -1;
+	return ~low; /* not found, return 1's complement of insert position */
 }
 
 /*
@@ -1213,39 +1229,46 @@ static int lookupenv(char **env, const char *name, size_t nmln)
  */
 static int do_putenv(char **env, const char *name, int size, int free_old)
 {
-	char *eq = strchrnul(name, '=');
-	int i = lookupenv(env, name, eq-name);
+	int i = bsearchenv(env, name, size - 1);
 
-	if (i < 0) {
-		if (*eq) {
-			env[size - 1] = (char*) name;
-			env[size] = NULL;
+	/* optionally free removed / replaced entry */
+	if (i >= 0 && free_old)
+		free(env[i]);
+
+	if (strchr(name, '=')) {
+		/* if new value ('key=value') is specified, insert or replace entry */
+		if (i < 0) {
+			i = ~i;
+			memmove(&env[i + 1], &env[i], (size - i) * sizeof(char*));
 			size++;
 		}
-	}
-	else {
-		if (free_old)
-			free(env[i]);
-		if (*eq)
-			env[i] = (char*) name;
-		else {
-			for (; env[i]; i++)
-				env[i] = env[i+1];
-			size--;
-		}
+		env[i] = (char*) name;
+	} else if (i >= 0) {
+		/* otherwise ('key') remove existing entry */
+		size--;
+		memmove(&env[i], &env[i + 1], (size - i) * sizeof(char*));
 	}
 	return size;
 }
 
-#undef getenv
+static char *do_getenv(const char *name)
+{
+	char *value;
+	int pos = bsearchenv(environ, name, environ_size - 1);
+	if (pos < 0)
+		return NULL;
+	value = strchr(environ[pos], '=');
+	return value ? &value[1] : NULL;
+}
+
 char *mingw_getenv(const char *name)
 {
-	char *result = getenv(name);
+	char *result = do_getenv(name);
 	if (!result && !strcmp(name, "TMPDIR")) {
 		/* on Windows it is TMP and TEMP */
-		result = getenv("TMP");
+		result = do_getenv("TMP");
 		if (!result)
-			result = getenv("TEMP");
+			result = do_getenv("TEMP");
 	}
 	return result;
 }
@@ -2088,6 +2111,9 @@ void mingw_startup()
 	environ[i] = NULL;
 	free(buffer);
 
+	/* sort environment for O(log n) getenv / putenv */
+	qsort(environ, i, sizeof(char*), compareenv);
+
 	/* initialize critical section for waitpid pinfo_t list */
 	InitializeCriticalSection(&pinfo_cs);