bugfix: ensure snippets end on a fresh line

this change ensures every snippet ends on a fresh line, ensuring
that the corresponding close snippet tag is written to its own line.
Without this, snippets which would end in the middle of a line would
ultimately cause the next parse to fail as the close tag would be
written on the snippet's last line, causing the snippet parser to
not identify the snippet end on next parse.

gcgen/generate.py

@@ -130,6 +130,7 @@ def on_snippet(
         scope["$snippets"] = self._snippets_scope.derive()
         try:
             snippet_fn(emitter, scope, snippet_arg)
+            emitter.freshline()
         except Exception as e:
             logger.error(
                 f"error executing snippet {snippet_name!r} in {fpath!s}", exc_info=True

tests/data/gentests/bb-snippets-always-end-last-line/input/gcgen_conf.py

@@ -0,0 +1,15 @@
+from gcgen.api import snippet, Emitter, Scope, Json
+
+
+@snippet("foo")
+def s_foo(e: Emitter, scope: Scope, val: Json):
+    e.emit("i end in the middle of a line...")
+
+
+@snippet("bar")
+def s_bar(e: Emitter, scope: Scope, val: Json):
+    e.emitln("I end on a fresh (new) line..")
+
+
+def gcgen_parse_files():
+    return ["testfile.txt"]

tests/data/gentests/bb-snippets-always-end-last-line/input/testfile.txt

@@ -0,0 +1,7 @@
+
+something
+# <<? foo
+# ?>>
+something else..
+# <<? bar
+# ?>>

tests/test_gentests.py

@@ -180,6 +180,19 @@ def test_bb_snippets_err_unclosed_snippet():
             generate.compile(gtc.input_path)
 
 
+def test_bb_snippets_always_end_last_line():
+    """encountered bug where a snippet which did not end on a fresh (empty)
+    line would cause the snippet end tag to be written on that line, causing
+    the next parse to fail because the corresponding snippet close tag was
+    not picked up.
+
+    This test is to ensure that we now always end on a fresh line, even if the
+    snippet last called `emit` rather than `emitln`."""
+    with load_gentest("bb-snippets-always-end-last-line") as gtc:
+        generate.compile(gtc.input_path)
+        generate.compile(gtc.input_path)
+
+
 def test_cc_generators_write_test():
     "write two files using a generator"
     gentest_test_eql("cc-generators-write-test", ["foo.txt", "bar.txt"])