sk_fgetsize to not use ftell.

The previous version of sk_fgetsize used ftell and fseek to compute
the size of a file. There are so many issues with this that it is called
out by securecoding.cert.org as FIO19-C as a thing not to do. We already
have correct code for computing the size of a file in the mmap code, so
use that instead.

Change-Id: I1d771124989d0ec1523f6d858814ee563263213a
Reviewed-on: https://skia-review.googlesource.com/9860
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>

src/ports/SkOSFile_posix.cpp

@@ -19,6 +19,25 @@
 #include <sys/types.h>
 #include <unistd.h>
 
+size_t sk_fgetsize(FILE* f) {
+    int fd = fileno(f);
+    if (fd < 0) {
+        return 0;
+    }
+
+    struct stat status;
+    if (0 != fstat(fd, &status)) {
+        return 0;
+    }
+    if (!S_ISREG(status.st_mode)) {
+        return 0;
+    }
+    if (!SkTFitsIn<size_t>(status.st_size)) {
+        return 0;
+    }
+    return static_cast<size_t>(status.st_size);
+}
+
 bool sk_exists(const char *path, SkFILE_Flags flags) {
     int mode = F_OK;
     if (flags & kRead_SkFILE_Flag) {

src/ports/SkOSFile_stdio.cpp

@@ -87,24 +87,6 @@ FILE* sk_fopen(const char path[], SkFILE_Flags flags) {
     return file;
 }
 
-size_t sk_fgetsize(FILE* f) {
-    SkASSERT(f);
-
-    long curr = ftell(f); // remember where we are
-    if (curr < 0) {
-        return 0;
-    }
-
-    fseek(f, 0, SEEK_END); // go to the end
-    long size = ftell(f); // record the size
-    if (size < 0) {
-        size = 0;
-    }
-
-    fseek(f, curr, SEEK_SET); // go back to our prev location
-    return size;
-}
-
 size_t sk_fwrite(const void* buffer, size_t byteCount, FILE* f) {
     SkASSERT(f);
     return fwrite(buffer, 1, byteCount, f);

src/ports/SkOSFile_win.cpp

@@ -16,6 +16,27 @@
 #include <stdio.h>
 #include <sys/stat.h>
 
+size_t sk_fgetsize(FILE* f) {
+    int fileno = sk_fileno(f);
+    if (fileno < 0) {
+        return 0;
+    }
+
+    HANDLE file = (HANDLE)_get_osfhandle(fileno);
+    if (INVALID_HANDLE_VALUE == file) {
+        return 0;
+    }
+
+    LARGE_INTEGER fileSize;
+    if (0 == GetFileSizeEx(file, &fileSize)) {
+        return 0;
+    }
+    if (!SkTFitsIn<size_t>(fileSize.QuadPart)) {
+        return 0;
+    }
+    return static_cast<size_t>(fileSize.QuadPart);
+}
+
 bool sk_exists(const char *path, SkFILE_Flags flags) {
     int mode = 0; // existence
     if (flags & kRead_SkFILE_Flag) {