Add gears.protected_call

This library is a wrapper around pcall() / xpcall() that prints an error message
via gears.debug.print_error() in case of errors.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/gears/init.lua

@@ -17,6 +17,7 @@ return
     cache = require("gears.cache");
     matrix = require("gears.matrix");
     shape = require("gears.shape");
+    protected_call = require("gears.protected_call");
 }
 
 -- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80

lib/gears/protected_call.lua

@@ -0,0 +1,58 @@
+---------------------------------------------------------------------------
+-- @author Uli Schlachter
+-- @copyright 2016 Uli Schlachter
+-- @release @AWESOME_VERSION@
+-- @module gears.protected_call
+---------------------------------------------------------------------------
+
+local gdebug = require("gears.debug")
+local tostring = tostring
+local traceback = debug.traceback
+local unpack = unpack or table.unpack -- luacheck: globals unpack (compatibility with Lua 5.1)
+local xpcall = xpcall
+
+local protected_call = {}
+
+local function error_handler(err)
+    gdebug.print_error(traceback("Error during a protected call: " .. tostring(err)))
+end
+
+local function handle_result(success, ...)
+    if success then
+        return ...
+    end
+end
+
+local do_pcall
+if _VERSION <= "Lua 5.1" then
+    -- Lua 5.1 doesn't support arguments in xpcall :-(
+    do_pcall = function(func, ...)
+        local args = { ... }
+        return handle_result(xpcall(function()
+            return func(unpack(args))
+        end, error_handler))
+    end
+else
+    do_pcall = function(func, ...)
+        return handle_result(xpcall(func, error_handler, ...))
+    end
+end
+
+--- Call a function in protected mode and handle error-reporting.
+-- If the function call succeeds, all results of the function are returned.
+-- Otherwise, an error message is printed and nothing is returned.
+-- @tparam function func The function to call
+-- @param ... Arguments to the function
+-- @return The result of the given function, or nothing if an error occurred.
+function protected_call.call(func, ...)
+    return do_pcall(func, ...)
+end
+
+local pcall_mt = {}
+function pcall_mt:__call(...)
+    return do_pcall(...)
+end
+
+return setmetatable(protected_call, pcall_mt)
+
+-- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80

spec/gears/protected_call_spec.lua

@@ -0,0 +1,43 @@
+---------------------------------------------------------------------------
+-- @author Uli Schlachter
+-- @copyright 2016 Uli Schlachter
+---------------------------------------------------------------------------
+
+local gdebug = require("gears.debug")
+local protected_call = require("gears.protected_call")
+
+describe("gears.protected_call", function()
+    -- Stop the error reporting during tests
+    local orig_print_error = gdebug.print_error
+    local errors
+    before_each(function()
+        errors = {}
+        gdebug.print_error = function(err)
+            table.insert(errors, err)
+        end
+    end)
+    after_each(function()
+        gdebug.print_error = orig_print_error
+    end)
+
+    it("Call with arguments and result", function()
+        local called = false
+        local function f(...)
+            called = true
+            assert.is_same({ ... }, { 1, "second" })
+            return "first", 2
+        end
+        local results = { protected_call(f, 1, "second") }
+        assert.is_true(called)
+        assert.is_same({ "first", 2 }, results)
+        assert.is_same(errors, {})
+    end)
+
+    it("Call with error", function()
+        assert.is_same({}, { protected_call(error, "I was called") })
+        assert.is_same(#errors, 1)
+        assert.is_truthy(string.find(errors[1], "I was called"))
+    end)
+end)
+
+-- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80