Add file access modes to sk_exists.

Both Windows and Posix 'access' calls take a mode parameter which,
in addition to checking existence, checks access modes.
This change exposes this functionality.

R=mtklein@google.com, reed@google.com

Author: bungeman@google.com

Review URL: https://codereview.chromium.org/384903002

include/core/SkOSFile.h

@@ -77,8 +77,10 @@ bool    sk_fidentical(SkFILE* a, SkFILE* b);
  */
 int     sk_fileno(SkFILE* f);
 
-// Returns true if something (file, directory, ???) exists at this path.
-bool    sk_exists(const char *path);
+/** Returns true if something (file, directory, ???) exists at this path,
+ *  and has the specified access flags.
+ */
+bool    sk_exists(const char *path, SkFILE_Flags = (SkFILE_Flags)0);
 
 // Returns true if a directory exists at this path.
 bool    sk_isdir(const char *path);

src/ports/SkOSFile_none.cpp

@@ -7,6 +7,10 @@
 
 #include "SkOSFile.h"
 
+bool sk_exists(const char *path, SkFILE_Flags flags) {
+    return false;
+}
+
 bool sk_fidentical(SkFILE* a, SkFILE* b) {
     return false;
 }

src/ports/SkOSFile_posix.cpp

@@ -13,6 +13,18 @@
 #include <sys/mman.h>
 #include <sys/stat.h>
 #include <sys/types.h>
+#include <unistd.h>
+
+bool sk_exists(const char *path, SkFILE_Flags flags) {
+    int mode = F_OK;
+    if (flags & kRead_SkFILE_Flag) {
+        mode |= R_OK;
+    }
+    if (flags & kWrite_SkFILE_Flag) {
+        mode |= W_OK;
+    }
+    return (0 == access(path, mode));
+}
 
 typedef struct {
     dev_t dev;

src/ports/SkOSFile_stdio.cpp

@@ -15,8 +15,6 @@
 #ifdef _WIN32
 #include <direct.h>
 #include <io.h>
-#else
-#include <unistd.h>
 #endif
 
 SkFILE* sk_fopen(const char path[], SkFILE_Flags flags) {
@@ -123,14 +121,6 @@ void sk_fclose(SkFILE* f) {
     ::fclose((FILE*)f);
 }
 
-bool sk_exists(const char *path) {
-#ifdef _WIN32
-    return (0 == _access(path, 0));
-#else
-    return (0 == access(path, 0));
-#endif
-}
-
 bool sk_isdir(const char *path) {
     struct stat status;
     if (0 != stat(path, &status)) {

src/ports/SkOSFile_win.cpp

@@ -13,6 +13,17 @@
 #include <stdio.h>
 #include <sys/stat.h>
 
+bool sk_exists(const char *path, SkFILE_Flags flags) {
+    int mode = 0; // existence
+    if (flags & kRead_SkFILE_Flag) {
+        mode |= 4; // read
+    }
+    if (flags & kWrite_SkFILE_Flag) {
+        mode |= 2; // write
+    }
+    return (0 == _access(path, mode));
+}
+
 typedef struct {
     ULONGLONG fVolume;
     ULONGLONG fLsbSize;