Support for O_TRUNC in the open() syscall (s3ql#197)

Changes.txt

@@ -1,3 +1,7 @@
+UNRELEASED CHANGES
+
+  * The open() syscall supports the O_TRUNC flag now.
+
 2020-09-04, S3QL 3.5.1
 
   * `s3qlctrl upload-meta` now works properly again (previously, only the first

src/s3ql/fs.py

@@ -58,6 +58,20 @@
     ACL_ERRNO = errno.EOPNOTSUPP
 
 
+class _TruncSetattrFields:
+    """A private class used for O_TRUNC.
+
+    This is needed because pyfuse3.SetattrFields fields are read only"""
+    def __getattr__(self, name):
+        """Only return ``True`` for ``update_size`` and ``update_mtime`.
+        Anything else will return ``False``"""
+        return name in ('update_size', 'update_mtime')
+
+
+"""We only need a single instance of this read-only class"""
+_TruncSetattrFields = _TruncSetattrFields()
+
+
 class Operations(pyfuse3.Operations):
     """A full-featured file system for online data storage
 
@@ -814,7 +828,7 @@ async def setattr(self, id_, attr, fields, fh, ctx):
         inode.ctime_ns = now_ns
 
         # This needs to go last, because the call to cache.remove and cache.get
-        # are asynchorouns and may thus evict the *inode* object from the cache.
+        # are asynchronous and may thus evict the *inode* object from the cache.
         if fields.update_size:
             len_ = attr.st_size
 
@@ -933,8 +947,20 @@ async def open(self, id_, flags, ctx):
             raise FUSEError(errno.EPERM)
 
         if flags & os.O_TRUNC:
-            log.warning('open() with O_TRUNC is not yet supported.')
-            raise FUSEError(errno.ENOTSUP)
+            if not (flags & os.O_RDWR or flags & os.O_WRONLY):
+                #  behaviour is not defined in POSIX, we opt for an error
+                raise FUSEError(errno.EINVAL)
+            attr = await self.getattr(id_, ctx)
+            if stat.S_ISREG(attr.st_mode):
+                attr.st_mtime_ns = time_ns()
+                attr.st_size = 0
+                await self.setattr(id_, attr, _TruncSetattrFields, None, ctx)
+            elif stat.S_ISFIFO(attr.st_mode) or stat.S_ISBLK(attr.st_mode):
+                #  silently ignore O_TRUNC when FIFO or terminal block device
+                pass
+            else:
+                #  behaviour is not defined in POSIX, we opt for an error
+                raise FUSEError(errno.EINVAL)
 
         return pyfuse3.FileInfo(fh=id_, keep_cache=True)
 

tests/t3_fs_api.py

@@ -552,7 +552,13 @@ async def test_truncate_0(ctx):
     attr.st_size = 0
     await ctx.server.setattr(inode.st_ino, attr, SetattrFields(update_size=True),
                         fh, some_ctx)
-    await ctx.server.write(fh, 0, random_data(len2))
+    data = random_data(len2)
+    await ctx.server.write(fh, 0, data)
+    await ctx.server.release(fh)
+    ctx.server.inodes.flush()
+    fi = await ctx.server.open(inode.st_ino, os.O_RDONLY, some_ctx)
+    fh = fi.fh
+    assert await ctx.server.read(fh, 0, len2 + 1) == data
     await ctx.server.release(fh)
     await ctx.server.forget([(inode.st_ino, 1)])
     await fsck(ctx)
@@ -816,6 +822,28 @@ async def test_create_open(ctx):
 
     await fsck(ctx)
 
+async def test_open_truncate(ctx):
+    (fi, inode) = await ctx.server.create(ROOT_INODE, newname(ctx), file_mode(),
+                                          os.O_RDWR, some_ctx)
+    fh = fi.fh
+    await ctx.server.write(fh, 0, random_data(int(1.5 * ctx.max_obj_size)))
+    await ctx.server.release(fh)
+    ctx.server.inodes.flush()
+
+    data = b'this data overwrites the previous data'
+    fi = await ctx.server.open(inode.st_ino, os.O_RDWR | os.O_TRUNC, some_ctx)
+    fh = fi.fh
+    await ctx.server.write(fh, 0, data)
+    await ctx.server.release(fh)
+    ctx.server.inodes.flush()
+
+    fi = await ctx.server.open(inode.st_ino, os.O_RDONLY, some_ctx)
+    fh = fi.fh
+    assert await ctx.server.read(fh, 0, len(data) + 1) == data
+    await ctx.server.release(fh)
+    await ctx.server.forget([(inode.st_ino, 1)])
+    await fsck(ctx)
+
 async def test_edit(ctx):
     len_ = ctx.max_obj_size
     data = random_data(len_)