diff options
| author | Pavan Deolasee | 2017-06-15 06:04:06 +0000 |
|---|---|---|
| committer | Pavan Deolasee | 2017-06-15 06:11:22 +0000 |
| commit | fb56418d660437d1cbaa0aa7a4260ca826792ec9 (patch) | |
| tree | e3612a55edb7b8ff23be61fa233b266498385cfa | |
| parent | 90d1109aa3397c6beb50b12cd8e5364861037424 (diff) | |
Use thread-specific storage for computing a snapshot.
We mustn't use a global variable since concurrent GTM threads might try to
compute a snapshot at the same time and may overwrite the information before a
thread can send the complete snapshot to the client. Chi Gao
<[email protected]> reported that this can cause infinite wait on the client
side because the client expects N bytes of data, but only receives (N - x)
bytes and it keeps waiting for remaining x bytes which the GTM never sends.
While we don't have a report, it's obvious that it can also go wrong in the
other direction.
We fix this by using a thread-specific storage which ensures that the snapshot
cannot be changed while it's being sent to the client.
Report, analysis and fix by Chi Gao <[email protected]>. Some minor
editorialisation by me.
Backpatched to XL9_5_STABLE
| -rw-r--r-- | src/gtm/main/gtm_snap.c | 11 | ||||
| -rw-r--r-- | src/include/gtm/gtm.h | 3 |
2 files changed, 10 insertions, 4 deletions
diff --git a/src/gtm/main/gtm_snap.c b/src/gtm/main/gtm_snap.c index f4ab3a127b..5a538985df 100644 --- a/src/gtm/main/gtm_snap.c +++ b/src/gtm/main/gtm_snap.c @@ -23,7 +23,6 @@ #include "gtm/libpq-int.h" #include "gtm/pqformat.h" -static GTM_SnapshotData localSnapshot; /* * Get snapshot for the given transactions. If this is the first call in the * transaction, a fresh snapshot is taken and returned back. For a serializable @@ -91,10 +90,16 @@ GTM_GetTransactionSnapshot(GTM_TransactionHandle handle[], int txn_count, int *s /* * If no valid transaction exists in the array, we record the snapshot in a - * local strucure and still send it out to the caller + * thread-specific structure. This allows us to avoid repeated + * allocation/freeing of the structure. + * + * Note that we must use a thread-specific variable and not a global + * variable because a concurrent thread might compute a new snapshot and + * overwrite the snapshot information while we are still sending this copy + * to the client. Using a thread-specific storage avoids that problem. */ if (snapshot == NULL) - snapshot = &localSnapshot; + snapshot = &GetMyThreadInfo->thr_snapshot; Assert(snapshot != NULL); diff --git a/src/include/gtm/gtm.h b/src/include/gtm/gtm.h index 8dfa77b139..37d31f6ba4 100644 --- a/src/include/gtm/gtm.h +++ b/src/include/gtm/gtm.h @@ -57,7 +57,8 @@ typedef struct GTM_ThreadInfo uint32 thr_client_id; /* unique client identifier */ GTM_RWLock thr_lock; - gtm_List *thr_cached_txninfo; + gtm_List *thr_cached_txninfo; + GTM_SnapshotData thr_snapshot; } GTM_ThreadInfo; typedef struct GTM_Threads |