Make reading from Pool more thread-safe

doc/libsolv-pool.txt

@@ -646,7 +646,10 @@ Utility functions
 
 Allocate space on the pool's temporary space area. This space has a limited
 lifetime, it will be automatically freed after a fixed amount (currently
-16) of other pool_alloctmpspace() calls are done.
+500) of other pool_alloctmpspace() calls are done.
+The function is not completely thread-safe, but it mitigates the problem by
+locking and using sufficiently large temporary space. That allows reading
+from pool from several tens of concurrent threads.
 
 	void pool_freetmpspace(Pool *pool, const char *space);
 

src/pool.c

@@ -1723,15 +1723,20 @@ pool_id2langid(Pool *pool, Id id, const char *lang, int create)
 char *
 pool_alloctmpspace(Pool *pool, int len)
 {
-  int n = pool->tmpspace.n;
   if (!len)
     return 0;
+
+  pthread_mutex_lock(&pool->tmpspace.lock);
+  int n = pool->tmpspace.n;
+  pool->tmpspace.n = (n + 1) % POOL_TMPSPACEBUF;
+  pthread_mutex_unlock(&pool->tmpspace.lock);
+
   if (len > pool->tmpspace.len[n])
     {
       pool->tmpspace.buf[n] = solv_realloc(pool->tmpspace.buf[n], len + 32);
       pool->tmpspace.len[n] = len + 32;
     }
-  pool->tmpspace.n = (n + 1) % POOL_TMPSPACEBUF;
+
   return pool->tmpspace.buf[n];
 }
 

src/pool.h

@@ -13,6 +13,7 @@
 #ifndef LIBSOLV_POOL_H
 #define LIBSOLV_POOL_H
 
+#include <pthread.h>
 #include <stdio.h>
 
 #include "solvversion.h"
@@ -53,12 +54,13 @@ typedef struct s_Datapos {
 #ifdef LIBSOLV_INTERNAL
 
 /* how many strings to maintain (round robin) */
-#define POOL_TMPSPACEBUF 16
+#define POOL_TMPSPACEBUF 500
 
 struct s_Pool_tmpspace {
   char *buf[POOL_TMPSPACEBUF];
   int   len[POOL_TMPSPACEBUF];
   int   n;
+  pthread_mutex_t lock;
 };
 
 #endif