Change. Write callback can return any true value (except numbers).

For example in Lua 5.2 file:write method returns `self`.

``` Lua
function write(str)
  return #str - 1 -- this is error (number less than #str)
end

function write(str)
  return {} -- this is pass
end
```

doc/lcurl.ldoc

@@ -228,7 +228,7 @@ function getinfo() end
 --
 -- A callback accepting one or two parameters.
 -- The first is the writer context if any, and the second is a string with the data to be written.
--- Function must return `true` or full data length or nothing to continue operation.
+-- Function must return `true` (any non number true value) or full data length or nothing to continue operation.
 -- Otherwise the transfer will be aborted with an error.
 --
 -- @tparam function writer
@@ -250,7 +250,7 @@ function setopt_writefunction() end
 --
 -- A callback accepting one or two parameters.
 -- The first is the writer context if any, and the second is a string with the data to be written.
--- Function must return `true` or full data length or nothing to continue operation.
+-- Function must return `true` (any non number true value) or full data length or nothing to continue operation.
 -- Otherwise the transfer will be aborted with an error.
 --
 -- @tparam function writer

src/lceasy.c

@@ -478,10 +478,12 @@ static int lcurl_write_callback_(lua_State*L,
 
   if(lua_gettop(L) > top){
     if(lua_isnil(L, top + 1)) return 0;
-    if(lua_isboolean(L, top + 1)){
+    if(lua_isnumber(L, top + 1)){
+      ret = (size_t)lua_tonumber(L, top + 1);
+    }
+    else{
       if(!lua_toboolean(L, top + 1)) ret = 0;
     }
-    else ret = (size_t)lua_tonumber(L, top + 1);
   }
 
   lua_settop(L, top);

test/run.lua

@@ -12,3 +12,4 @@ print("------------------------------------")
 print("")
 
 require "test_safe"
+require "test_easy"

test/test_easy.lua

@@ -0,0 +1,48 @@
+local HAS_RUNNER = not not lunit
+local lunit      = require "lunit"
+local TEST_CASE  = assert(lunit.TEST_CASE)
+local skip       = lunit.skip or function() end
+
+local curl       = require "lcurl"
+local scurl      = require "lcurl.safe"
+local url        = "http://example.com"
+local fname      = "./test.download"
+
+local _ENV = TEST_CASE'write_callback' do
+
+local c, f
+
+function teardown()
+  if f then f:close() end
+  os.remove(fname)
+  if c then c:close() end
+  f, c = nil
+end
+
+function test_write_to_file()
+  f = assert(io.open(fname, "w+b"))
+  c = assert(curl.easy{
+    url = url;
+    writefunction = f;
+  })
+
+  assert_equal(c, c:perform())
+end
+
+function test_write_to_file_abort()
+  f = assert(io.open(fname, "w+b"))
+  c = assert(scurl.easy{
+    url = url;
+    writefunction = function(str)
+      return #str - 1
+    end;
+  })
+
+  local _, e = assert_nil(c:perform())
+  assert_equal(e, curl.error(curl.ERROR_EASY, curl.E_WRITE_ERROR))
+
+end
+
+end
+
+if not HAS_RUNNER then lunit.run() end