Resolve immutable snippet sources

scripts/snippets/resolution.py

@@ -0,0 +1,39 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Protocol
+
+from .model import ImmutableSourceReference
+
+DEFAULT_MAX_SOURCE_BYTES = 1024 * 1024
+
+
+class SourceResolutionError(Exception):
+    """A snippet source could not be resolved safely."""
+
+
+@dataclass(frozen=True)
+class ResolvedSource:
+    reference: ImmutableSourceReference
+    commit: str
+    content: bytes
+
+
+class GitHubFileClient(Protocol):
+    def read_file(self, repository: str, commit: str, path: str) -> bytes: ...
+
+
+def resolve_immutable_source(
+    reference: ImmutableSourceReference,
+    github: GitHubFileClient,
+    *,
+    max_source_bytes: int = DEFAULT_MAX_SOURCE_BYTES,
+) -> ResolvedSource:
+    """Read an immutable source at the exact commit declared by the page."""
+
+    content = github.read_file(reference.repository, reference.commit, reference.path)
+    if len(content) > max_source_bytes:
+        raise SourceResolutionError(
+            f"Source exceeds the {max_source_bytes}-byte size limit"
+        )
+    return ResolvedSource(reference, reference.commit, content)

tests/test_immutable_snippet_source_resolution.py

@@ -0,0 +1,46 @@
+from __future__ import annotations
+
+import pytest
+
+from scripts.snippets.model import ImmutableSourceReference
+from scripts.snippets.resolution import (
+    SourceResolutionError,
+    resolve_immutable_source,
+)
+
+COMMIT = "2c941ea9e834d7602d388f3271c0f864025ea756"
+
+
+class FakeGitHub:
+    def __init__(self, content: bytes = b"hello\n") -> None:
+        self.content = content
+        self.calls: list[tuple[str, str, str]] = []
+
+    def read_file(self, repository: str, commit: str, path: str) -> bytes:
+        self.calls.append((repository, commit, path))
+        return self.content
+
+
+def reference() -> ImmutableSourceReference:
+    return ImmutableSourceReference(
+        repository="canton-network/splice",
+        commit=COMMIT,
+        path="apps/example.yaml",
+    )
+
+
+def test_reads_the_exact_declared_commit() -> None:
+    github = FakeGitHub()
+
+    resolved = resolve_immutable_source(reference(), github)
+
+    assert resolved.commit == COMMIT
+    assert resolved.content == b"hello\n"
+    assert github.calls == [
+        ("canton-network/splice", COMMIT, "apps/example.yaml")
+    ]
+
+
+def test_rejects_source_larger_than_the_configured_limit() -> None:
+    with pytest.raises(SourceResolutionError, match="4-byte size limit"):
+        resolve_immutable_source(reference(), FakeGitHub(b"12345"), max_source_bytes=4)