[PATCH v4 0/8] fast-import: tighten parsing of paths

Updated to address Junio's review comments. Thanks!

This round needed no code changes, so is probably ready, if the documentation changes look good.

Changes since v3:
* Reword documentation paragraphs on path quoting.
* Fix minor typo in commit message.

Thalia

Thalia Archibald (8):
  fast-import: tighten path unquoting
  fast-import: directly use strbufs for paths
  fast-import: allow unquoted empty path for root
  fast-import: remove dead strbuf
  fast-import: improve documentation for path quoting
  fast-import: document C-style escapes for paths
  fast-import: forbid escaped NUL in paths
  fast-import: make comments more precise

 Documentation/git-fast-import.txt |  31 +-
 builtin/fast-import.c             | 158 ++++----
 t/t9300-fast-import.sh            | 624 +++++++++++++++++++++---------
 3 files changed, 551 insertions(+), 262 deletions(-)

Range-diff against v3:
1:  d9ab0c6a75 = 1:  d6ea8aca46 fast-import: tighten path unquoting
2:  696ca27bb7 = 2:  9499f34aae fast-import: directly use strbufs for paths
3:  39879d0a66 ! 3:  9b1e6b80f5 fast-import: allow unquoted empty path for root
    @@ Commit message
         also be an empty string (`""`) to specify the root of the tree” to “The
         root of the tree can be represented by an empty string as `<path>`”.
     
    -    Thus, we can assume that some front-ends have depended on this behavior.
    +    Thus, we should assume that some front-ends have depended on this
    +    behavior.
     
         Remove this restriction for the destination paths of filecopy and
         filerename and change tests targeting the root to test `""` and ``.
4:  1cef05e59a = 4:  1a2b0dc616 fast-import: remove dead strbuf
5:  2e78690023 ! 5:  fb0d870d53 fast-import: improve documentation for unquoted paths
    @@ Metadata
     Author: Thalia Archibald <thalia@archibald.dev>
     
      ## Commit message ##
    -    fast-import: improve documentation for unquoted paths
    +    fast-import: improve documentation for path quoting
     
    -    It describes what cannot be in an unquoted path, but not what it is.
    -    Reframe it as a definition of unquoted paths. The requirement that it
    -    not start with `"` is the core element that implies the rest.
    +    It describes what characters cannot be in an unquoted path, but not
    +    their semantics. Reframe it as a definition of unquoted paths. From the
    +    perspective of the parser, whether it starts with `"` is what defines
    +    whether it will parse it as quoted or unquoted.
    +
    +    The restrictions on characters in unquoted paths (with starting-", LF,
    +    and spaces) are explained in the quoted paragraph. Move it to the
    +    unquoted paragraph and reword.
     
         The restriction that the source paths of filecopy and filerename cannot
         contain SP is only stated in their respective sections. Restate it in
    @@ Documentation/git-fast-import.txt: in octal.  Git only supports the following mo
     -A `<path>` string must use UNIX-style directory separators (forward
     -slash `/`), may contain any byte other than `LF`, and must not
     -start with double quote (`"`).
    -+A `<path>` can be written as unquoted bytes or a C-style quoted string:
    ++A `<path>` can be written as unquoted bytes or a C-style quoted string.
      
     -A path can use C-style string quoting; this is accepted in all cases
     -and mandatory if the filename starts with double quote or contains
     -`LF`. In C-style quoting, the complete name should be surrounded with
    -+When a `<path>` does not start with double quote (`"`), it is an
    +-double quotes, and any `LF`, backslash, or double quote characters
    +-must be escaped by preceding them with a backslash (e.g.,
    +-`"path/with\n, \\ and \" in it"`).
    ++When a `<path>` does not start with a double quote (`"`), it is an
     +unquoted string and is parsed as literal bytes without any escape
     +sequences. However, if the filename contains `LF` or starts with double
    -+quote, it must be written as a quoted string. Additionally, the source
    -+`<path>` in `filecopy` or `filerename` must be quoted if it contains SP.
    -+
    -+A `<path>` can use C-style string quoting; this is accepted in all cases
    -+and mandatory in the cases where the filename cannot be represented as
    -+an unquoted string. In C-style quoting, the complete name should be surrounded with
    - double quotes, and any `LF`, backslash, or double quote characters
    - must be escaped by preceding them with a backslash (e.g.,
    - `"path/with\n, \\ and \" in it"`).
    ++quote, it cannot be represented as an unquoted string and must be
    ++quoted. Additionally, the source `<path>` in `filecopy` or `filerename`
    ++must be quoted if it contains SP.
      
     -The value of `<path>` must be in canonical form. That is it must not:
    ++When a `<path>` starts with a double quote (`"`), it is a C-style quoted
    ++string, where the complete filename is enclosed in a pair of double
    ++quotes and escape sequences are used. Certain characters must be escaped
    ++by preceding them with a backslash: `LF` is written as `\n`, backslash
    ++as `\\`, and double quote as `\"`. All filenames can be represented as
    ++quoted strings.
    ++
     +A `<path>` must use UNIX-style directory separators (forward slash `/`)
    -+and must be in canonical form. That is it must not:
    ++and its value must be in canonical form. That is it must not:
      
      * contain an empty directory component (e.g. `foo//bar` is invalid),
      * end with a directory separator (e.g. `foo/` is invalid),
6:  1b07ddffe0 ! 6:  4b6017ded8 fast-import: document C-style escapes for paths
    @@ Commit message
         Signed-off-by: Thalia Archibald <thalia@archibald.dev>
     
      ## Documentation/git-fast-import.txt ##
    -@@ Documentation/git-fast-import.txt: quote, it must be written as a quoted string. Additionally, the source
    - 
    - A `<path>` can use C-style string quoting; this is accepted in all cases
    - and mandatory in the cases where the filename cannot be represented as
    --an unquoted string. In C-style quoting, the complete name should be surrounded with
    --double quotes, and any `LF`, backslash, or double quote characters
    --must be escaped by preceding them with a backslash (e.g.,
    --`"path/with\n, \\ and \" in it"`).
    -+an unquoted string. In C-style quoting, the complete filename is
    -+surrounded with double quote (`"`) and certain characters must be
    -+escaped by preceding them with a backslash: `LF` is written as `\n`,
    -+backslash as `\\`, and double quote as `\"`. Some characters may may
    -+optionally be written with escape sequences: `\a` for bell, `\b` for
    -+backspace, `\f` for form feed, `\n` for line feed, `\r` for carriage
    -+return, `\t` for horizontal tab, and `\v` for vertical tab. Any byte can
    -+be written with 3-digit octal codes (e.g., `\033`).
    +@@ Documentation/git-fast-import.txt: When a `<path>` starts with a double quote (`"`), it is a C-style quoted
    + string, where the complete filename is enclosed in a pair of double
    + quotes and escape sequences are used. Certain characters must be escaped
    + by preceding them with a backslash: `LF` is written as `\n`, backslash
    +-as `\\`, and double quote as `\"`. All filenames can be represented as
    ++as `\\`, and double quote as `\"`. Some characters may optionally be
    ++written with escape sequences: `\a` for bell, `\b` for backspace, `\f`
    ++for form feed, `\n` for line feed, `\r` for carriage return, `\t` for
    ++horizontal tab, and `\v` for vertical tab. Any byte can be written with
    ++3-digit octal codes (e.g., `\033`). All filenames can be represented as
    + quoted strings.
      
      A `<path>` must use UNIX-style directory separators (forward slash `/`)
    - and must be in canonical form. That is it must not:
     
      ## t/t9300-fast-import.sh ##
     @@ t/t9300-fast-import.sh: test_path_eol_success () {
7:  dc67464b6a = 7:  5b464f4b01 fast-import: forbid escaped NUL in paths
8:  5e02d887bc = 8:  6eb66fce45 fast-import: make comments more precise