Add BufFileRead variants with short read and EOF detection

Most callers of BufFileRead() want to check whether they read the full
specified length.  Checking this at every call site is very tedious.
This patch provides additional variants BufFileReadExact() and
BufFileReadMaybeEOF() that include the length checks.

I considered changing BufFileRead() itself, but this function is also
used in extensions, and so changing the behavior like this would
create a lot of problems there.  The new names are analogous to the
existing LogicalTapeReadExact().

Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/f3501945-c591-8cc3-5ef0-b72a2e0eaa9c@enterprisedb.com

src/backend/storage/file/buffile.c

@@ -573,14 +573,19 @@ BufFileDumpBuffer(BufFile *file)
 }
 
 /*
- * BufFileRead
+ * BufFileRead variants
  *
  * Like fread() except we assume 1-byte element size and report I/O errors via
  * ereport().
+ *
+ * If 'exact' is true, then an error is also raised if the number of bytes
+ * read is not exactly 'size' (no short reads).  If 'exact' and 'eofOK' are
+ * true, then reading zero bytes is ok.
  */
-size_t
-BufFileRead(BufFile *file, void *ptr, size_t size)
+static size_t
+BufFileReadCommon(BufFile *file, void *ptr, size_t size, bool exact, bool eofOK)
 {
+	size_t		start_size = size;
 	size_t		nread = 0;
 	size_t		nthistime;
 
@@ -612,9 +617,48 @@ BufFileRead(BufFile *file, void *ptr, size_t size)
 		nread += nthistime;
 	}
 
+	if (exact &&
+		(nread != start_size && !(nread == 0 && eofOK)))
+		ereport(ERROR,
+				errcode_for_file_access(),
+				file->name ?
+				errmsg("could not read from file set \"%s\": read only %zu of %zu bytes",
+					   file->name, nread, start_size) :
+				errmsg("could not read from temporary file: read only %zu of %zu bytes",
+					   nread, start_size));
+
 	return nread;
 }
 
+/*
+ * Legacy interface where the caller needs to check for end of file or short
+ * reads.
+ */
+size_t
+BufFileRead(BufFile *file, void *ptr, size_t size)
+{
+	return BufFileReadCommon(file, ptr, size, false, false);
+}
+
+/*
+ * Require read of exactly the specified size.
+ */
+void
+BufFileReadExact(BufFile *file, void *ptr, size_t size)
+{
+	BufFileReadCommon(file, ptr, size, true, false);
+}
+
+/*
+ * Require read of exactly the specified size, but optionally allow end of
+ * file (in which case 0 is returned).
+ */
+size_t
+BufFileReadMaybeEOF(BufFile *file, void *ptr, size_t size, bool eofOK)
+{
+	return BufFileReadCommon(file, ptr, size, true, eofOK);
+}
+
 /*
  * BufFileWrite
  *