[profile] Factor out logic for mmap'ing merged profile, NFC

Split out the logic to get the size of a merged profile and to do a
compatibility check. This can be shared with both the continuous+merging
mode implementation, as well as the runtime-allocated counters
implementation planned for Fuchsia.

Lifted out of D69586.

Differential Revision: https://reviews.llvm.org/D70135

Author: vedantk

compiler-rt/lib/profile/InstrProfilingFile.c

@@ -171,21 +171,18 @@ static void setupIOBuffer() {
   }
 }
 
-/* Read profile data in \c ProfileFile and merge with in-memory
-   profile counters. Returns -1 if there is fatal error, otheriwse
-   0 is returned. Returning 0 does not mean merge is actually
-   performed. If merge is actually done, *MergeDone is set to 1.
-*/
-static int doProfileMerging(FILE *ProfileFile, int *MergeDone) {
-  uint64_t ProfileFileSize;
-  char *ProfileBuffer;
-
+/* Get the size of the profile file. If there are any errors, print the
+ * message under the assumption that the profile is being read for merging
+ * purposes, and return -1. Otherwise return the file size in the inout param
+ * \p ProfileFileSize. */
+static int getProfileFileSizeForMerging(FILE *ProfileFile,
+                                        uint64_t *ProfileFileSize) {
   if (fseek(ProfileFile, 0L, SEEK_END) == -1) {
     PROF_ERR("Unable to merge profile data, unable to get size: %s\n",
              strerror(errno));
     return -1;
   }
-  ProfileFileSize = ftell(ProfileFile);
+  *ProfileFileSize = ftell(ProfileFile);
 
   /* Restore file offset.  */
   if (fseek(ProfileFile, 0L, SEEK_SET) == -1) {
@@ -194,28 +191,60 @@ static int doProfileMerging(FILE *ProfileFile, int *MergeDone) {
     return -1;
   }
 
-  /* Nothing to merge.  */
-  if (ProfileFileSize < sizeof(__llvm_profile_header)) {
-    if (ProfileFileSize)
-      PROF_WARN("Unable to merge profile data: %s\n",
-                "source profile file is too small.");
-    return 0;
+  if (*ProfileFileSize > 0 &&
+      *ProfileFileSize < sizeof(__llvm_profile_header)) {
+    PROF_WARN("Unable to merge profile data: %s\n",
+              "source profile file is too small.");
+    return -1;
   }
+  return 0;
+}
 
-  ProfileBuffer = mmap(NULL, ProfileFileSize, PROT_READ, MAP_SHARED | MAP_FILE,
-                       fileno(ProfileFile), 0);
-  if (ProfileBuffer == MAP_FAILED) {
+/* mmap() \p ProfileFile for profile merging purposes, assuming that an
+ * exclusive lock is held on the file and that \p ProfileFileSize is the
+ * length of the file. Return the mmap'd buffer in the inout variable
+ * \p ProfileBuffer. Returns -1 on failure. On success, the caller is
+ * responsible for unmapping the mmap'd buffer in \p ProfileBuffer. */
+static int mmapProfileForMerging(FILE *ProfileFile, uint64_t ProfileFileSize,
+                                 char **ProfileBuffer) {
+  *ProfileBuffer = mmap(NULL, ProfileFileSize, PROT_READ, MAP_SHARED | MAP_FILE,
+                        fileno(ProfileFile), 0);
+  if (*ProfileBuffer == MAP_FAILED) {
     PROF_ERR("Unable to merge profile data, mmap failed: %s\n",
              strerror(errno));
     return -1;
   }
 
-  if (__llvm_profile_check_compatibility(ProfileBuffer, ProfileFileSize)) {
-    (void)munmap(ProfileBuffer, ProfileFileSize);
+  if (__llvm_profile_check_compatibility(*ProfileBuffer, ProfileFileSize)) {
+    (void)munmap(*ProfileBuffer, ProfileFileSize);
     PROF_WARN("Unable to merge profile data: %s\n",
               "source profile file is not compatible.");
-    return 0;
+    return -1;
   }
+  return 0;
+}
+
+/* Read profile data in \c ProfileFile and merge with in-memory
+   profile counters. Returns -1 if there is fatal error, otheriwse
+   0 is returned. Returning 0 does not mean merge is actually
+   performed. If merge is actually done, *MergeDone is set to 1.
+*/
+static int doProfileMerging(FILE *ProfileFile, int *MergeDone) {
+  uint64_t ProfileFileSize;
+  char *ProfileBuffer;
+
+  /* Get the size of the profile on disk. */
+  if (getProfileFileSizeForMerging(ProfileFile, &ProfileFileSize) == -1)
+    return -1;
+
+  /* Nothing to merge.  */
+  if (!ProfileFileSize)
+    return 0;
+
+  /* mmap() the profile and check that it is compatible with the data in
+   * the current image. */
+  if (mmapProfileForMerging(ProfileFile, ProfileFileSize, &ProfileBuffer) == -1)
+    return -1;
 
   /* Now start merging */
   __llvm_profile_merge_from_buffer(ProfileBuffer, ProfileFileSize);