log: measure the debug log with a size type that can hold it

GetFilesize() returned int while ftell() returns long, so a debug.log of
2 GiB or more came back negative. Its only caller is the -maxdebugfilesize
roll added in ae1c6a7, which skips the roll on a negative size: the
rotation written for issue #34 would have switched itself off permanently
at exactly the size it exists to prevent. Production was at 1.95 GiB
growing 51-129 MB/day, about a day short of crossing 2 GiB.

The same truncation capped the comparison at INT_MAX (2047.99 MB), so
-maxdebugfilesize=2048 or higher could never be exceeded and disabled
rotation outright. Both failures were silent.

Widen the return to int64 and seek with ftello/fseeko (_ftelli64 and
_fseeki64 on Windows). The roll in util.h is the function's only caller
in the tree, so no other behaviour changes. A 32-bit off_t would
reintroduce the same truncation, so its width is now a compile-time
assertion rather than a silent runtime surprise.

1 files changed, 30 insertions, 6 deletions

diff --git a/src/util.cpp b/src/util.cpp
index a5a4e80..60142be 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -415,13 +415,37 @@ bool FileExists(const char* psz)
 #endif
 }
 
-int GetFilesize(FILE* file)
+#ifndef _WIN32
+// If off_t is 32 bits wide, ftello() truncates exactly like the int this
+// function used to return. Fail the build rather than ship the silent
+// version of the bug (32-bit hosts need -D_FILE_OFFSET_BITS=64).
+static_assert(sizeof(off_t) >= 8, "GetFilesize needs a 64-bit off_t");
+#endif
+
+// Returns the file's size in bytes, or -1 if it cannot be determined.
+//
+// This used to hold ftell()'s long in an int, so a file of 2 GiB or more came
+// back negative. Its only caller is the debug.log roll in util.h, which skips
+// the roll on a negative size -- i.e. rotation switched itself off for good at
+// exactly the size the roll exists to prevent, and any cap of 2048 MB or more
+// could never be exceeded at all. Both failures were silent.
+int64 GetFilesize(FILE* file)
 {
-    int nSavePos = ftell(file);
-    int nFilesize = -1;
-    if (fseek(file, 0, SEEK_END) == 0)
-        nFilesize = ftell(file);
-    fseek(file, nSavePos, SEEK_SET);
+#ifdef _WIN32
+    __int64 nSavePos = _ftelli64(file);
+    int64 nFilesize = -1;
+    if (nSavePos >= 0 && _fseeki64(file, 0, SEEK_END) == 0)
+        nFilesize = _ftelli64(file);
+    if (nSavePos >= 0)
+        _fseeki64(file, nSavePos, SEEK_SET);
+#else
+    off_t nSavePos = ftello(file);
+    int64 nFilesize = -1;
+    if (nSavePos >= 0 && fseeko(file, 0, SEEK_END) == 0)
+        nFilesize = ftello(file);
+    if (nSavePos >= 0)
+        fseeko(file, nSavePos, SEEK_SET);
+#endif
     return nFilesize;
 }