Fix dynahash's HASH_FIXED_SIZE ("isfixed") option.

This flag was effectively a no-op in EXEC_BACKEND (ie, Windows)
builds, because it was kept in the process-local HTAB struct,
and it could only ever become set in the postmaster's copy.

The simplest fix is to move it to the shared HASHHDR struct.
We could keep a copy in HTAB as well, as we do with keysize
and some other fields, but the "too much contention" argument
doesn't seem to apply here: we only examine isfixed during
element_alloc(), which had better not get hit very often for
a shared hashtable.

This oversight dates to 7c797e719 which invented the option.
But back-patching doesn't seem appropriate given the lack of
field complaints.  If there is anyone running an affected
workload on Windows, they might be unhappy about the behavior
changing in a minor release.

Author: Aidar Imamov <a.imamov@postgrespro.ru>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/4d0cb35ff01c5c74d2b9a582ecb73823@postgrespro.ru

diff --git a/src/backend/utils/hash/dynahash.c b/src/backend/utils/hash/dynahash.c
index 1ad155d446e51d4eae551370228dad202cc54852..42e9be274fc6a08a1d0b651e4bfe099923c057d3 100644 (file)
--- a/src/backend/utils/hash/dynahash.c
+++ b/src/backend/utils/hash/dynahash.c
@@ -195,6 +195,7 @@ struct HASHHDR
        long            ssize;                  /* segment size --- must be power of 2 */
        int                     sshift;                 /* segment shift = log2(ssize) */
        int                     nelem_alloc;    /* number of entries to allocate at once */
+       bool            isfixed;                /* if true, don't enlarge */
 
 #ifdef HASH_STATISTICS
 
@@ -227,7 +228,6 @@ struct HTAB
        MemoryContext hcxt;                     /* memory context if default allocator used */
        char       *tabname;            /* table name (for error messages) */
        bool            isshared;               /* true if table is in shared memory */
-       bool            isfixed;                /* if true, don't enlarge */
 
        /* freezing a shared table isn't allowed, so we can keep state here */
        bool            frozen;                 /* true = no more inserts allowed */
@@ -618,8 +618,10 @@ hash_create(const char *tabname, long nelem, const HASHCTL *info, int flags)
                }
        }
 
+       /* Set isfixed if requested, but not till after we build initial entries */
        if (flags & HASH_FIXED_SIZE)
-               hashp->isfixed = true;
+               hctl->isfixed = true;
+
        return hashp;
 }
 
@@ -644,6 +646,8 @@ hdefault(HTAB *hashp)
        hctl->ssize = DEF_SEGSIZE;
        hctl->sshift = DEF_SEGSIZE_SHIFT;
 
+       hctl->isfixed = false;          /* can be enlarged */
+
 #ifdef HASH_STATISTICS
        hctl->accesses = hctl->collisions = 0;
 #endif
@@ -1713,7 +1717,7 @@ element_alloc(HTAB *hashp, int nelem, int freelist_idx)
        HASHELEMENT *prevElement;
        int                     i;
 
-       if (hashp->isfixed)
+       if (hctl->isfixed)
                return false;
 
        /* Each element has a HASHELEMENT header plus user data. */