Fail requests that are searching for a non existing position (#2938)

### Motivation

This is an attempt to handle and better understand #2446

We're seeing the server get stuck sometimes and I suspect that what's happening is the following:

1. A position request gets triggered, like completion, looking for a certain spot in the code
2. The code changes right as we start locating the position
3. Ruby switches threads and we process the text edit **before** finishing finding the specified position
4. Since now the document is in a different state, that position is no longer valid and it may lead to infinite loops

I propose we start raising when we fail to locate a position in the document and then we fail the request. This will hopefully help us better understand what's happening.

### Implementation

Started raising if the scanner position is already past the document range, which may happen if we modify the document exactly as we're searching for the position.

### Automated Tests

Added tests.

lib/ruby_lsp/document.rb

@@ -15,6 +15,7 @@ class LanguageId < T::Enum
     extend T::Helpers
     extend T::Generic
 
+    class LocationNotFoundError < StandardError; end
     ParseResultType = type_member
 
     # This maximum number of characters for providing expensive features, like semantic highlighting and diagnostics.
@@ -144,7 +145,15 @@ def initialize(source, encoding)
       def find_char_position(position)
         # Find the character index for the beginning of the requested line
         until @current_line == position[:line]
-          @pos += 1 until LINE_BREAK == @source[@pos]
+          until LINE_BREAK == @source[@pos]
+            @pos += 1
+
+            if @pos >= @source.length
+              # Pack the code points back into the original string to provide context in the error message
+              raise LocationNotFoundError, "Requested position: #{position}\nSource:\n\n#{@source.pack("U*")}"
+            end
+          end
+
           @pos += 1
           @current_line += 1
         end

lib/ruby_lsp/server.rb

@@ -119,12 +119,24 @@ def process_message(message)
         # If a document is deleted before we are able to process all of its enqueued requests, we will try to read it
         # from disk and it raise this error. This is expected, so we don't include the `data` attribute to avoid
         # reporting these to our telemetry
-        if e.is_a?(Store::NonExistingDocumentError)
+        case e
+        when Store::NonExistingDocumentError
           send_message(Error.new(
             id: message[:id],
             code: Constant::ErrorCodes::INVALID_PARAMS,
             message: e.full_message,
           ))
+        when Document::LocationNotFoundError
+          send_message(Error.new(
+            id: message[:id],
+            code: Constant::ErrorCodes::REQUEST_FAILED,
+            message: <<~MESSAGE,
+              Request #{message[:method]} failed to find the target position.
+              The file might have been modified while the server was in the middle of searching for the target.
+              If you experience this regularly, please report any findings and extra information on
+              https://github.com/Shopify/ruby-lsp/issues/2446
+            MESSAGE
+          ))
         else
           send_message(Error.new(
             id: message[:id],

test/ruby_document_test.rb

@@ -770,6 +770,29 @@ class Foo
     assert_nil(document.cache_get("textDocument/codeLens"))
   end
 
+  def test_locating_a_non_existing_location_raises
+    document = RubyLsp::RubyDocument.new(source: <<~RUBY.chomp, version: 1, uri: URI("file:///foo/bar.rb"))
+      class Foo
+      end
+    RUBY
+
+    # Exactly at the last character doesn't raise
+    document.locate_node({ line: 1, character: 2 })
+
+    # Anything beyond does
+    error = assert_raises(RubyLsp::Document::LocationNotFoundError) do
+      document.locate_node({ line: 3, character: 2 })
+    end
+
+    assert_equal(<<~MESSAGE.chomp, error.message)
+      Requested position: {:line=>3, :character=>2}
+      Source:
+
+      class Foo
+      end
+    MESSAGE
+  end
+
   private
 
   def assert_error_edit(actual, error_range)

test/server_test.rb

@@ -714,6 +714,36 @@ def handle_window_show_message_response(title)
     end
   end
 
+  def test_requests_to_a_non_existing_position_return_error
+    uri = URI("file:///foo.rb")
+
+    @server.process_message({
+      method: "textDocument/didOpen",
+      params: {
+        textDocument: {
+          uri: uri,
+          text: "class Foo\nend",
+          version: 1,
+          languageId: "ruby",
+        },
+      },
+    })
+
+    @server.process_message({
+      id: 1,
+      method: "textDocument/completion",
+      params: {
+        textDocument: {
+          uri: uri,
+        },
+        position: { line: 10, character: 0 },
+      },
+    })
+
+    error = find_message(RubyLsp::Error)
+    assert_match("Request textDocument/completion failed to find the target position.", error.message)
+  end
+
   private
 
   def with_uninstalled_rubocop(&block)