Cleanup: Replace sscanf with strtol/strtoul in snapmgr

On 4/20/26 07:06, Amul Sul wrote:

The attached patch replaces sscanf with strtol and strtoul in the
ImportSnapshot helpers (parseIntFromText, parseXidFromText, and
parseVxidFromText) to improve reliability and efficiency. By utilizing
the end pointer, we can locate the next line without re-scanning the
entire string.

Additionally, this change aligns the snapshot code with the rest of
the Postgres backend, which already favors these functions for safer
parsing.

I personally prefer this safer and easier to verify parsing so from me
this is a +1. I also reviewed the patch and it is simple, looks like it
handles errors correctly and matches code we have in other parts of our
code so I am all for merging it in its current shape. It also preserves
the old behavior of ignoring random stuff at the end of each line, for
good and bad.

Looks good to me!

Andreas

On Mon Apr 20, 2026 at 12:07 AM CDT, Amul Sul wrote:

Hi,

The attached patch replaces sscanf with strtol and strtoul in the
ImportSnapshot helpers (parseIntFromText, parseXidFromText, and
parseVxidFromText) to improve reliability and efficiency. By utilizing
the end pointer, we can locate the next line without re-scanning the
entire string.

Additionally, this change aligns the snapshot code with the rest of
the Postgres backend, which already favors these functions for safer
parsing.

Hey Amul,

The patch generally looks good. One comment:

@@ -1359,17 +1365,36 @@ parseVxidFromText(const char *prefix, char **s, const char *filename,
{
char       *ptr = *s;
int                     prefixlen = strlen(prefix);
+       long            lval;
+       unsigned long ulval;

Perhaps better variable names would be procNumber and
localTransactionId.

+ char *endptr;

if (strncmp(ptr, prefix, prefixlen) != 0)
ereport(ERROR,
(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
errmsg("invalid snapshot data in file \"%s\"", filename)));
ptr += prefixlen;
-       if (sscanf(ptr, "%d/%u", &vxid->procNumber, &vxid->localTransactionId) != 2)
+
+       /* Parse procNumber (the signed integer before '/') */
+       errno = 0;
+       lval = strtol(ptr, &endptr, 10);
+       if (endptr == ptr || errno != 0 || lval < INT_MIN || lval > INT_MAX ||
+               *endptr != '/')
ereport(ERROR,
(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
errmsg("invalid snapshot data in file \"%s\"", filename)));
-       ptr = strchr(ptr, '\n');
+       vxid->procNumber = (ProcNumber) lval;
+       ptr = endptr + 1;                       /* skip the '/' separator */
+
+       /* Parse localTransactionId (the unsigned integer after '/') */
+       errno = 0;
+       ulval = strtoul(ptr, &endptr, 10);
+       if (endptr == ptr || errno != 0 || ulval > UINT_MAX)
+               ereport(ERROR,
+                               (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+                                errmsg("invalid snapshot data in file \"%s\"", filename)));
+       vxid->localTransactionId = (LocalTransactionId) ulval;
+       ptr = strchr(endptr, '\n');
if (!ptr)
ereport(ERROR,
(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),

Otherwise, this looks committable to me. In reviewing, I learned that
sscanf() will parse a string like " 45" as 45, so doesn't seem like we
will have any behavioral differences using strto*().

--
Tristan Partin
PostgreSQL Contributors Team
AWS (https://aws.amazon.com)

On Mon, May 4, 2026 at 9:19 PM Tristan Partin <tristan@partin.io> wrote:

On Mon Apr 20, 2026 at 12:07 AM CDT, Amul Sul wrote:
The patch generally looks good. One comment:

@@ -1359,17 +1365,36 @@ parseVxidFromText(const char *prefix, char **s, const char *filename,
{
char       *ptr = *s;
int                     prefixlen = strlen(prefix);
+       long            lval;
+       unsigned long ulval;

Perhaps better variable names would be procNumber and
localTransactionId.

Thanks, Andreas and Tristan, for the review !

I have renamed the variables as suggested but used the shorter forms
procno and xid instead of procNumber and localTransactionId. I also
applied similar changes to parseXidFromText (changing val to xid), but
kept val in parseIntFromText since it seems to be more appropriate for
a generic integer value.

Updated patch attached.

Regards,
Amul

On Tue May 5, 2026 at 2:24 AM CDT, Amul Sul wrote:

On Mon, May 4, 2026 at 9:19 PM Tristan Partin <tristan@partin.io> wrote:

On Mon Apr 20, 2026 at 12:07 AM CDT, Amul Sul wrote:
The patch generally looks good. One comment:

@@ -1359,17 +1365,36 @@ parseVxidFromText(const char *prefix, char **s, const char *filename,
{
char       *ptr = *s;
int                     prefixlen = strlen(prefix);
+       long            lval;
+       unsigned long ulval;

Perhaps better variable names would be procNumber and
localTransactionId.

Thanks, Andreas and Tristan, for the review !

I have renamed the variables as suggested but used the shorter forms
procno and xid instead of procNumber and localTransactionId. I also
applied similar changes to parseXidFromText (changing val to xid), but
kept val in parseIntFromText since it seems to be more appropriate for
a generic integer value.

Updated patch attached.

New patch looks good to me. I can confirm that the only changes in the
new version of the patch are the variable names.

--
Tristan Partin
PostgreSQL Contributors Team
AWS (https://aws.amazon.com)