lua: handle uri.format empty input properly

After 7fd6c809
(buffer: port static allocator to Lua) uri started to use
static_allocator - cyclic buffer that also is used in
several modules.

However situation when uri.format output is zero-length
string was not handled properly and ffi.string could
return data that was previously written in static buffer
because use as string terminator the first zero byte.

To prevent such situation let's pass result length explicitly.

Closes #4779

(cherry picked from commit 57f6fc93)

src/lua/uri.lua

@@ -77,8 +77,8 @@ local function format(uri, write_password)
     uribuf.fragment = uri.fragment
     uribuf.fragment_len = string.len(uri.fragment or '')
     local str = static_alloc('char', 1024)
-    builtin.uri_format(str, 1024, uribuf, write_password and 1 or 0)
-    return ffi.string(str)
+    local len = builtin.uri_format(str, 1024, uribuf, write_password and 1 or 0)
+    return ffi.string(str, len)
 end
 
 return {

test/app-tap/uri.test.lua

@@ -2,6 +2,8 @@
 
 local tap = require('tap')
 local uri = require('uri')
+local ffi = require('ffi')
+local static_alloc = require('buffer').static_alloc
 
 local function test_parse(test)
     -- Tests for uri.parse() Lua bindings.
@@ -64,8 +66,28 @@ local function test_format(test)
     test:is(uri.format(u, true), "user:password@localhost", "password kept")
 end
 
+local function test_static_alloc(test)
+    -- gh-4779 uri.format returns junk output.
+    -- As static allocator is also used in several places
+    -- we should properly handle situation when output
+    -- is zero-length string.
+    -- Here we allocate the whole buffer,
+    -- fill it with some "junk" bytes and
+    -- check that result doesn't contain any of them.
+    local buffer_size = 12288
+    local buf = static_alloc('char', buffer_size)
+    ffi.fill(buf, 512, string.byte('x'))
+
+    local iterations = 10
+    test:plan(iterations)
+    for _ = 1, iterations do
+        test:is(uri.format({}), '')
+    end
+end
+
 tap.test("uri", function(test)
-    test:plan(2)
+    test:plan(3)
     test:test("parse", test_parse)
     test:test("format", test_format)
+    test:test("static_alloc", test_static_alloc)
 end)