Add test case for fiber safety of digest.pbkdf2

Follows up #3396.

test/app/digest.result

@@ -5,6 +5,9 @@ test_run:cmd("push filter ".."'\\.lua.*:[0-9]+: ' to '.lua:<line>\"]: '")
 ---
 - true
 ...
+fiber = require('fiber')
+---
+...
 digest = require('digest')
 ---
 ...
@@ -576,3 +579,60 @@ test_run:cmd("clear filter")
 ---
 - true
 ...
+-- gh-3396: fiber-safe pbkdf2
+res = {}
+---
+...
+sentry = fiber.channel()
+---
+...
+_ = test_run:cmd("setopt delimiter ';'")
+---
+...
+function test_pbkdf2()
+    local digest = require('digest')
+    for i = 1, 10 do
+        table.insert(res, digest.pbkdf2('', 'salt', 100, 32):hex())
+    end
+    sentry:put(fiber.id())
+end;
+---
+...
+_ = test_run:cmd("setopt delimiter ''");
+---
+...
+_ = fiber.create(test_pbkdf2)
+---
+...
+_ = fiber.create(test_pbkdf2)
+---
+...
+_ = sentry:get()
+---
+...
+_ = sentry:get()
+---
+...
+res
+---
+- - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+  - bafac115a0022b2894f2983b5b5102455bdd3ba7cfbeb09f219a9fde8f3ee6a9
+...

test/app/digest.test.lua

 test_run = require('test_run').new()
 test_run:cmd("push filter ".."'\\.lua.*:[0-9]+: ' to '.lua:<line>\"]: '")
 
+fiber = require('fiber')
 digest = require('digest')
 type(digest)
 
@@ -184,3 +185,21 @@ s
 err:match("number")
 digest = nil
 test_run:cmd("clear filter")
+
+-- gh-3396: fiber-safe pbkdf2
+res = {}
+sentry = fiber.channel()
+_ = test_run:cmd("setopt delimiter ';'")
+function test_pbkdf2()
+    local digest = require('digest')
+    for i = 1, 10 do
+        table.insert(res, digest.pbkdf2('', 'salt', 100, 32):hex())
+    end
+    sentry:put(fiber.id())
+end;
+_ = test_run:cmd("setopt delimiter ''");
+_ = fiber.create(test_pbkdf2)
+_ = fiber.create(test_pbkdf2)
+_ = sentry:get()
+_ = sentry:get()
+res