import: No empty commits without --allow-empty

This adds the --allow-empty option, which has the same meaning as
in `commit` and `apply`.

This behaviour mostly only affects `--replace-existing`,
since `import` will never produce an empty commit if you're not
replacing anything.

sno/fast_import.py

@@ -1,13 +1,14 @@
 import logging
 import subprocess
 import time
+import uuid
 from enum import Enum, auto
 
 import click
 import pygit2
 
 from . import git_util
-from .exceptions import SubprocessError, InvalidOperation
+from .exceptions import SubprocessError, InvalidOperation, NotFound, NO_CHANGES
 from .import_source import ImportSource
 from .structure import DatasetStructure, RepositoryStructure
 from .repository_version import get_repo_version, extra_blobs_for_version
@@ -38,6 +39,7 @@ def fast_import_tables(
     header=None,
     message=None,
     replace_existing=ReplaceExisting.DONT_REPLACE,
+    allow_empty=False,
     limit=None,
     max_pack_size="2G",
     max_delta_depth=0,
@@ -87,16 +89,26 @@ def fast_import_tables(
         f"--depth={max_delta_depth}",
     ] + list(extra_cmd_args)
 
+    orig_branch = get_head_branch(repo)
     if header is None:
-        header = generate_header(repo, sources, message)
+        # import onto a temp branch. then reset the head branch afterwards.
+        # this allows us to check the result before updating the orig branch.
+        import_branch = f'refs/heads/{uuid.uuid4()}'
+        header = generate_header(repo, sources, message, branch=import_branch)
+    else:
+        import_branch = orig_branch
 
     if not quiet:
         click.echo("Starting git-fast-import...")
 
-    p = subprocess.Popen(cmd, cwd=repo.path, stdin=subprocess.PIPE,)
+    p = subprocess.Popen(
+        cmd,
+        cwd=repo.path,
+        stdin=subprocess.PIPE,
+    )
     try:
         if replace_existing != ReplaceExisting.ALL:
-            header += f"from {get_head_branch(repo)}^0\n"
+            header += f"from {orig_branch}^0\n"
         p.stdin.write(header.encode("utf8"))
 
         # Write any extra blobs supplied by the client or needed for this version.
@@ -183,6 +195,23 @@ def fast_import_tables(
     if not quiet:
         click.echo(f"Closed in {(t3-t2):.0f}s")
 
+    if import_branch != orig_branch:
+        try:
+            if head_tree and not allow_empty:
+                if repo.revparse_single(import_branch).peel(pygit2.Tree) == head_tree:
+                    raise NotFound("No changes to commit", exit_code=NO_CHANGES)
+            # reset the original branch head to the import branch, so it gets the new commits
+            if head_tree:
+                # repo was non-empty before this, so orig_branch exists already.
+                # we have to delete and re-create it at the new commit.
+                repo.references.delete(orig_branch)
+            repo.references.create(
+                orig_branch, repo.references[import_branch].peel(pygit2.Commit).oid
+            )
+        finally:
+            # remove the import branch
+            repo.references.delete(import_branch)
+
 
 def get_head_tree(repo):
     """Returns the tree at the current repo HEAD."""
@@ -214,14 +243,16 @@ def write_blobs_to_stream(stream, blobs):
         yield i, blob_path
 
 
-def generate_header(repo, sources, message):
+def generate_header(repo, sources, message, branch=None):
     if message is None:
         message = generate_message(sources)
 
     author = git_util.author_signature(repo)
     committer = git_util.committer_signature(repo)
+    if branch is None:
+        branch = get_head_branch(repo)
     return (
-        f"commit {get_head_branch(repo)}\n"
+        f"commit {branch}\n"
         f"author {author.name} <{author.email}> {author.time} {minutes_to_tz_offset(author.offset)}\n"
         f"committer {committer.name} <{committer.email}> {committer.time} {minutes_to_tz_offset(committer.offset)}\n"
         f"data {len(message.encode('utf8'))}\n{message}\n"

sno/init.py

@@ -83,7 +83,8 @@ def temporary_branch(repo):
 @click.pass_context
 @click.argument("source")
 @click.argument(
-    "tables", nargs=-1,
+    "tables",
+    nargs=-1,
 )
 @click.option(
     "--all-tables",
@@ -141,7 +142,10 @@ def temporary_branch(repo):
     help="List available import formats, and then exit",
 )
 @click.option(
-    "--output-format", "-o", type=click.Choice(["text", "json"]), default="text",
+    "--output-format",
+    "-o",
+    type=click.Choice(["text", "json"]),
+    default="text",
 )
 @click.option(
     "--primary-key",
@@ -152,6 +156,16 @@ def temporary_branch(repo):
     is_flag=True,
     help="Replace existing dataset(s) of the same name.",
 )
+@click.option(
+    "--allow-empty",
+    is_flag=True,
+    default=False,
+    help=(
+        "Usually recording a commit that has the exact same tree as its sole "
+        "parent commit is a mistake, and the command prevents you from making "
+        "such a commit. This option bypasses the safety"
+    ),
+)
 @click.option(
     "--max-delta-depth",
     hidden=True,
@@ -170,6 +184,7 @@ def import_table(
     tables,
     table_info,
     replace_existing,
+    allow_empty,
     max_delta_depth,
 ):
     """
@@ -261,6 +276,7 @@ def import_table(
             replace_existing=ReplaceExisting.GIVEN
             if replace_existing
             else ReplaceExisting.DONT_REPLACE,
+            allow_empty=allow_empty,
         )
 
     rs = RepositoryStructure(repo)
@@ -314,7 +330,14 @@ def import_table(
     help="--depth option to git-fast-import (advanced users only)",
 )
 def init(
-    ctx, message, directory, repo_version, import_from, bare, wc_path, max_delta_depth,
+    ctx,
+    message,
+    directory,
+    repo_version,
+    import_from,
+    bare,
+    wc_path,
+    max_delta_depth,
 ):
     """
     Initialise a new repository and optionally import data.
@@ -347,7 +370,10 @@ def init(
 
     if import_from:
         fast_import_tables(
-            repo, sources, message=message, max_delta_depth=max_delta_depth,
+            repo,
+            sources,
+            message=message,
+            max_delta_depth=max_delta_depth,
         )
         head_commit = repo.head.peel(pygit2.Commit)
         checkout.reset_wc_if_needed(repo, head_commit)

tests/test_init.py

@@ -89,7 +89,10 @@ def test_import_table_with_prompt(data_archive_readonly, tmp_path, cli_runner, c
         assert r.exit_code == 0
         with chdir(repo_path):
             r = cli_runner.invoke(
-                ["import", data / "census2016_sdhca_ot_short.gpkg",],
+                [
+                    "import",
+                    data / "census2016_sdhca_ot_short.gpkg",
+                ],
                 input="census2016_sdhca_ot_ced_short\n",
             )
             # Table was specified interactively via prompt
@@ -178,7 +181,11 @@ def test_import_table_with_prompt_with_no_input(
 
 
 def test_import_replace_existing(
-    data_archive, tmp_path, cli_runner, chdir, geopackage,
+    data_archive,
+    tmp_path,
+    cli_runner,
+    chdir,
+    geopackage,
 ):
     with data_archive("gpkg-polygons") as data:
         repo_path = tmp_path / 'emptydir'
@@ -237,8 +244,45 @@ def test_import_replace_existing(
             }
 
 
+def test_import_replace_existing_with_no_changes(
+    data_archive,
+    tmp_path,
+    cli_runner,
+    chdir,
+    geopackage,
+):
+    with data_archive("gpkg-polygons") as data:
+        repo_path = tmp_path / 'emptydir'
+        r = cli_runner.invoke(["init", repo_path])
+        assert r.exit_code == 0
+        with chdir(repo_path):
+            r = cli_runner.invoke(
+                [
+                    "import",
+                    data / "nz-waca-adjustments.gpkg",
+                    "nz_waca_adjustments:mytable",
+                ]
+            )
+            assert r.exit_code == 0, r.stderr
+
+            # now import the same thing over the top (no changes)
+            r = cli_runner.invoke(
+                [
+                    "import",
+                    "--replace-existing",
+                    data / "nz-waca-adjustments.gpkg",
+                    "nz_waca_adjustments:mytable",
+                ]
+            )
+            assert r.exit_code == 44, r.stderr
+
+
 def test_import_replace_existing_with_compatible_schema_changes(
-    data_archive, tmp_path, cli_runner, chdir, geopackage,
+    data_archive,
+    tmp_path,
+    cli_runner,
+    chdir,
+    geopackage,
 ):
     with data_archive("gpkg-polygons") as data:
         repo_path = tmp_path / 'emptydir'
@@ -304,7 +348,11 @@ def test_import_replace_existing_with_compatible_schema_changes(
 
 
 def test_import_replace_existing_with_column_renames(
-    data_archive, tmp_path, cli_runner, chdir, geopackage,
+    data_archive,
+    tmp_path,
+    cli_runner,
+    chdir,
+    geopackage,
 ):
     with data_archive("gpkg-polygons") as data:
         repo_path = tmp_path / 'emptydir'
@@ -594,7 +642,11 @@ def test_init_import(
 
 
 def test_init_import_commit_headers(
-    data_archive, tmp_path, cli_runner, chdir, geopackage,
+    data_archive,
+    tmp_path,
+    cli_runner,
+    chdir,
+    geopackage,
 ):
     with data_archive("gpkg-points") as data:
         repo_path = tmp_path / "data.sno"