From 0fce3212a3eadb904fecda557b72c16ed822a75c Mon Sep 17 00:00:00 2001
From: Jonathan Coates <git@squiddev.cc>
Date: Sat, 4 Mar 2023 10:29:52 +0000
Subject: [PATCH] Distinguish between all parsers passing and failing

Given an input like f(x), which is both a valid statement and
expression, both parsers would accept the whole input. However, this was
treated the same as both parsers rejecting the input, resulting in a
crash when trying to print the error.

We now return immediately when any parser accepts the input.

Fixes #1354
---
 .../modules/main/cc/internal/syntax/init.lua  | 23 +++++++++++++++----
 .../modules/cc/internal/syntax/init_spec.lua  | 14 +++++++++++
 2 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/src/main/resources/data/computercraft/lua/rom/modules/main/cc/internal/syntax/init.lua b/src/main/resources/data/computercraft/lua/rom/modules/main/cc/internal/syntax/init.lua
index 6b52f1ce0..00df37a24 100644
--- a/src/main/resources/data/computercraft/lua/rom/modules/main/cc/internal/syntax/init.lua
+++ b/src/main/resources/data/computercraft/lua/rom/modules/main/cc/internal/syntax/init.lua
@@ -120,22 +120,35 @@ local function parse_repl(input)
         assert(coroutine.resume(parsers[i], context, coroutine.yield, start_code))
     end
 
+    -- Run all parsers together in parallel, feeding them one token at a time.
+    -- Once all parsers have failed, report the last failure (corresponding to
+    -- the longest parse).
     local ok, err = pcall(function()
         local parsers_n = #parsers
         while true do
             local token, start, finish = lexer()
 
-            local stop = true
+            local all_failed = true
             for i = 1, parsers_n do
                 local parser = parsers[i]
-                if coroutine.status(parser) ~= "dead" then
-                    stop = false
+                if parser then
                     local ok, err = coroutine.resume(parser, token, start, finish)
-                    if not ok and err ~= error_sentinel then error(err, 0) end
+                    if ok then
+                        -- This parser accepted our input, succeed immediately.
+                        if coroutine.status(parser) == "dead" then return end
+
+                        all_failed = false -- Otherwise continue parsing.
+                    elseif err ~= error_sentinel then
+                        -- An internal error occurred: propagate it.
+                        error(err, 0)
+                    else
+                        -- The parser failed, stub it out so we don't try to continue using it.
+                        parsers[i] = false
+                    end
                 end
             end
 
-            if stop then error(error_sentinel) end
+            if all_failed then error(error_sentinel) end
         end
     end)
 
diff --git a/src/test/resources/test-rom/spec/modules/cc/internal/syntax/init_spec.lua b/src/test/resources/test-rom/spec/modules/cc/internal/syntax/init_spec.lua
index 69bddaa89..10ae30f78 100644
--- a/src/test/resources/test-rom/spec/modules/cc/internal/syntax/init_spec.lua
+++ b/src/test/resources/test-rom/spec/modules/cc/internal/syntax/init_spec.lua
@@ -49,4 +49,18 @@ describe("cc.internal.syntax", function()
     describe_golden("the lexer", "lexer_spec.md", true)
     describe_golden("the parser", "parser_spec.md", false)
     describe_golden("the parser (all states)", "parser_exhaustive_spec.md", false)
+
+    describe("the REPL input parser", function()
+        it("returns true when accepted by both parsers", function()
+            helpers.with_window(50, 10, function()
+                expect(syntax.parse_repl("print(x)")):eq(true)
+            end)
+        end)
+
+        it("returns true when accepted by one parser", function()
+            helpers.with_window(50, 10, function()
+                expect(syntax.parse_repl("x")):eq(true)
+            end)
+        end)
+    end)
 end)