Avoid pathological behavior in OpenBSD's fread.

(cherry picked from commit 20841a137b)

Bug: https://code.google.com/p/android/issues/detail?id=81155
Bug: 18556607
Change-Id: Ibdfebc20dce4c34ad565014523c9b074e90ea665

libc/Android.mk

@@ -61,6 +61,7 @@ libc_common_src_files := \
     bionic/sigsetmask.c \
     bionic/system_properties_compat.c \
     stdio/findfp.c \
+    stdio/fread.c \
     stdio/snprintf.c\
     stdio/sprintf.c \
 
@@ -396,7 +397,6 @@ libc_upstream_openbsd_src_files := \
     upstream-openbsd/lib/libc/stdio/fputs.c \
     upstream-openbsd/lib/libc/stdio/fputwc.c \
     upstream-openbsd/lib/libc/stdio/fputws.c \
-    upstream-openbsd/lib/libc/stdio/fread.c \
     upstream-openbsd/lib/libc/stdio/freopen.c \
     upstream-openbsd/lib/libc/stdio/fscanf.c \
     upstream-openbsd/lib/libc/stdio/fseek.c \

libc/stdio/fread.c

@@ -68,7 +68,23 @@ fread(void *buf, size_t size, size_t count, FILE *fp)
 		fp->_r = 0;
 	total = resid;
 	p = buf;
-	while (resid > (r = fp->_r)) {
+
+	// BEGIN android-added
+	// Avoid pathological behavior on unbuffered files. OpenBSD
+	// will loop reading one byte then memcpying one byte!
+	if ((fp->_flags & __SNBF) != 0) {
+		// We know if we're unbuffered that our buffer is empty, so
+		// we can just read directly.
+		while (resid > 0 && (r = (*fp->_read)(fp->_cookie, p, resid)) > 0) {
+			p += r;
+			resid -= r;
+		}
+		FUNLOCKFILE(fp);
+		return ((total - resid) / size);
+	}
+	// END android-added
+
+	while (resid > (size_t)(r = fp->_r)) {
 		(void)memcpy((void *)p, (void *)fp->_p, (size_t)r);
 		fp->_p += r;
 		/* fp->_r = 0 ... done in __srefill */

tests/stdio_test.cpp

@@ -694,3 +694,34 @@ TEST(stdio, fpos_t_and_seek) {
 
   fclose(fp);
 }
+
+// https://code.google.com/p/android/issues/detail?id=81155
+// http://b/18556607
+TEST(stdio, fread_unbuffered_pathological_performance) {
+  FILE* fp = fopen("/dev/zero", "r");
+  ASSERT_TRUE(fp != NULL);
+
+  // Make this stream unbuffered.
+  setvbuf(fp, 0, _IONBF, 0);
+
+  char buf[65*1024];
+  memset(buf, 0xff, sizeof(buf));
+
+  time_t t0 = time(NULL);
+  for (size_t i = 0; i < 1024; ++i) {
+    fread(buf, 64*1024, 1, fp);
+  }
+  time_t t1 = time(NULL);
+
+  fclose(fp);
+
+  // 1024 64KiB reads should have been very quick.
+  ASSERT_LE(t1 - t0, 1);
+
+  for (size_t i = 0; i < 64*1024; ++i) {
+    ASSERT_EQ('\0', buf[i]);
+  }
+  for (size_t i = 64*1024; i < 65*1024; ++i) {
+    ASSERT_EQ('\xff', buf[i]);
+  }
+}