130 lines
4.7 KiB
Python
130 lines
4.7 KiB
Python
"""The content-addressed blob store: tiers 2 and 3 on disk.
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One store for both, and docs/architecture.md says why in a sentence: once tier 3
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is decoded by the app rather than by a shell script, frames and audio become "the
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same kind of thing as tier 2 — a cache with a hash". So there is one place that
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writes bytes, one that reads them, and one URL shape for both.
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TWO KINDS OF HASH, AND THEY ARE NOT THE SAME HASH. A blob is named by the sha256
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of its BYTES: that is what makes identical frames in two extractions one file. A
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derived thing — an analysis artifact, a dense block — is named by a sha256 over
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its INPUTS, which is what lets the client ask for the block the current settings
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want before anything has computed it. So `Block.key` is an input hash and
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`Block.data.digest` is a byte hash, and conflating them would break the half of
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addressing that answers questions about work not yet done.
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"""
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import hashlib
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import os
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import tempfile
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from pathlib import Path
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from django.conf import settings
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CHUNK = 1 << 20
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def digest_bytes(data: bytes) -> str:
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return hashlib.sha256(data).hexdigest()
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def digest_file(path: Path) -> str:
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h = hashlib.sha256()
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with open(path, "rb") as fh:
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while chunk := fh.read(CHUNK):
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h.update(chunk)
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return h.hexdigest()
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def path_for(digest: str) -> Path:
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"""Where a blob lives.
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Fanned out two levels, so that a take's worth of frames does not put a hundred
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thousand entries in one directory — which is slow on every filesystem and
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unusable on some.
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"""
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if len(digest) != 64 or any(c not in "0123456789abcdef" for c in digest):
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raise ValueError(f"not a sha256: {digest!r}")
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return Path(settings.BLOB_ROOT) / digest[:2] / digest[2:4] / digest
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def write(data: bytes) -> tuple[str, int]:
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"""Store bytes, return (digest, size). Writing the same bytes twice is a
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no-op, which is what content addressing is for."""
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digest = digest_bytes(data)
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dest = path_for(digest)
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if not dest.exists():
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dest.parent.mkdir(parents=True, exist_ok=True)
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tmp = dest.with_suffix(".part")
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with open(tmp, "wb") as fh:
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fh.write(data)
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os.replace(tmp, dest)
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return digest, len(data)
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def write_stream(chunks) -> tuple[str, int]:
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"""Store an uploaded file without reading the whole video into memory."""
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root = Path(settings.BLOB_ROOT)
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root.mkdir(parents=True, exist_ok=True)
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digest = hashlib.sha256()
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size = 0
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with tempfile.NamedTemporaryFile(dir=root, prefix="upload-", delete=False) as out:
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temporary = Path(out.name)
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try:
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for chunk in chunks:
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digest.update(chunk)
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size += len(chunk)
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out.write(chunk)
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except BaseException:
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temporary.unlink(missing_ok=True)
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raise
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dest = path_for(digest.hexdigest())
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dest.parent.mkdir(parents=True, exist_ok=True)
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if dest.exists():
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temporary.unlink()
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else:
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os.replace(temporary, dest)
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return digest.hexdigest(), size
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def adopt(source: Path) -> tuple[str, int]:
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"""Store a file already on disk, by hard link where the filesystem allows it.
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112MB of PNGs is a normal extraction and copying them into a second place in
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the tree for no reason is not. A hard link is exact — the blob is immutable, so
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two names for one inode is the whole of what is wanted — and a copy is the
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fallback when `extract.sh` wrote to another volume.
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"""
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digest = digest_file(source)
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dest = path_for(digest)
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size = source.stat().st_size
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if not dest.exists():
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dest.parent.mkdir(parents=True, exist_ok=True)
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try:
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os.link(source, dest)
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except OSError:
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tmp = dest.with_suffix(".part")
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with open(source, "rb") as src, open(tmp, "wb") as out:
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while chunk := src.read(CHUNK):
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out.write(chunk)
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os.replace(tmp, dest)
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return digest, size
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def read(digest: str) -> bytes:
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with open(path_for(digest), "rb") as fh:
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return fh.read()
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def png_size(path: Path) -> tuple[int, int]:
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"""A PNG's dimensions, out of its IHDR.
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Twenty-four bytes rather than a dependency. The footage's width and height are
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manifest data — docs/architecture.md's entity model puts them there — and
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Pillow to read two integers out of a header that has held them in the same
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place since 1996 is not a trade worth making.
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"""
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with open(path, "rb") as fh:
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head = fh.read(24)
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if head[:8] != b"\x89PNG\r\n\x1a\n" or head[12:16] != b"IHDR":
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raise ValueError(f"{path} is not a PNG")
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return int.from_bytes(head[16:20], "big"), int.from_bytes(head[20:24], "big")
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