util/init_random_seed: write_file implies deterministic

This makes Zeek run in deterministic mode with --save-seeds usage
and reworks all the extra indirections used in init_random_seed()
to make it easier to follow the control flow.

Fixes #4209

NEWS

@@ -27,6 +27,10 @@ Changed Functionality
   then prompt to use --help. The --help usage will now print to standard output
   rather than standard error.
 
+- Saving seeds with ``--save-seeds`` will now put Zeek into deterministic mode.
+  A subsequent ``--load-seeds`` run with the same scripts and traces will produce
+  identical UID values as the original ``--save-seeds` run.
+
 Removed Functionality
 ---------------------
 

src/util.cc

@@ -363,9 +363,8 @@ static long int zeek_rand_state = 0;
 static bool first_seed_saved = false;
 static unsigned int first_seed = 0;
 
-static void zeek_srandom(unsigned int seed, bool deterministic) {
+static void zeek_srandom(unsigned int seed) {
     zeek_rand_state = seed == 0 ? 1 : seed;
-    zeek_rand_deterministic = deterministic;
 
     srandom(seed);
 }
@@ -380,26 +379,28 @@ void seed_random(unsigned int seed) {
 void init_random_seed(const char* read_file, const char* write_file, bool use_empty_seeds,
                       const std::string& seed_string) {
     std::array<uint32_t, zeek::detail::KeyedHash::SEED_INIT_SIZE> buf = {};
-    size_t pos = 0; // accumulates entropy
-    bool seeds_done = false;
     uint32_t seed = 0;
 
+    if ( write_file )
+        // run in deterministic mode when we write a file
+        zeek_rand_deterministic = true;
+
+    if ( read_file || use_empty_seeds || ! seed_string.empty() ) {
+        // if a seed is provided - run Zeek in deterministic mode
+        zeek_rand_deterministic = true;
+
         if ( read_file ) {
             if ( ! read_random_seeds(read_file, &seed, buf) )
                 reporter->FatalError("Could not load seeds from file '%s'.", read_file);
-        else
-            seeds_done = true;
         }
         else if ( ! seed_string.empty() ) {
             if ( ! fill_random_seeds(seed_string, &seed, buf) )
                 reporter->FatalError("Could not load seeds from string");
-        else
-            seeds_done = true;
         }
-    else if ( use_empty_seeds )
-        seeds_done = true;
+    }
+    else {              // no seed provided
+        size_t pos = 0; // accumulates entropy
 
-    if ( ! seeds_done ) {
 #ifdef HAVE_GETRANDOM
         // getrandom() guarantees reads up to 256 bytes are always successful,
         assert(sizeof(buf) < 256);
@@ -437,17 +438,13 @@ void init_random_seed(const char* read_file, const char* write_file, bool use_em
             reporter->FatalError("Could not read enough random data. Wanted %d, got %zu",
                                  zeek::detail::KeyedHash::SEED_INIT_SIZE, pos);
 
-        if ( ! seed ) {
         for ( size_t i = 0; i < pos; ++i ) {
             seed ^= buf[i];
             seed = (seed << 1) | (seed >> 31);
         }
     }
-        else
-            seeds_done = true;
-    }
 
-    zeek_srandom(seed, seeds_done);
+    zeek_srandom(seed);
 
     if ( ! first_seed_saved ) {
         first_seed = seed;

testing/btest/Baseline/core.save-load-seeds/load.conn.log.cut

@@ -0,0 +1,3 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+uid	history	service
+CHhAvVGS1DHFjwGM9	ShADadFf	http

testing/btest/Baseline/core.save-load-seeds/save.conn.log.cut

@@ -0,0 +1,3 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+uid	history	service
+CHhAvVGS1DHFjwGM9	ShADadFf	http

testing/btest/core/save-load-seeds.zeek

@@ -0,0 +1,16 @@
+# @TEST-DOC: Save seeds and read and assure the UIDs are the same. Regression test for #4209
+#
+# @TEST-EXEC: zeek --save-seeds myseeds -r $TRACES/http/get.trace %INPUT
+# @TEST-EXEC: mkdir save && mv *log save
+# @TEST-EXEC: zeek-cut -m uid history service < save/conn.log >save/conn.log.cut
+#
+# @TEST-EXEC: zeek --load-seeds myseeds -r $TRACES/http/get.trace %INPUT
+# @TEST-EXEC: mkdir load && mv *log load
+# @TEST-EXEC: zeek-cut -m uid history service < load/conn.log >load/conn.log.cut
+#
+# @TEST-EXEC: btest-diff load/conn.log.cut
+# @TEST-EXEC: btest-diff save/conn.log.cut
+# @TEST-EXEC: diff load/conn.log.cut save/conn.log.cut
+
+@load base/protocols/conn
+@load base/protocols/http