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Re: [PATCH v5 1/3] ls-refs: report unborn targets of symrefs

From
Junio C Hamano <gitster@pobox.com>
Date
Jan 26, 2021, 21:38 UTC
Message-ID
<xmqq8s8f1iqe.fsf@gitster.c.googlers.com>
In-Reply-To
<32e16dfdbd3f54c0d4b39cbd555e66aa3950fffd.1611686656.git.jonathantanmy@google.com>
Jonathan Tan <jonathantanmy@google.com> writes:
Show 111 quoted lines
> When cloning, we choose the default branch based on the remote HEAD.
> But if there is no remote HEAD reported (which could happen if the
> target of the remote HEAD is unborn), we'll fall back to using our local
> init.defaultBranch. Traditionally this hasn't been a big deal, because
> most repos used "master" as the default. But these days it is likely to
> cause confusion if the server and client implementations choose
> different values (e.g., if the remote started with "main", we may choose
> "master" locally, create commits there, and then the user is surprised
> when they push to "master" and not "main").
>
> To solve this, the remote needs to communicate the target of the HEAD
> symref, even if it is unborn, and "git clone" needs to use this
> information.
>
> Currently, symrefs that have unborn targets (such as in this case) are
> not communicated by the protocol. Teach Git to advertise and support the
> "unborn" feature in "ls-refs" (by default, this is advertised, but
> server administrators may turn this off through the lsrefs.allowunborn
> config). This feature indicates that "ls-refs" supports the "unborn"
> argument; when it is specified, "ls-refs" will send the HEAD symref with
> the name of its unborn target.
>
> This change is only for protocol v2. A similar change for protocol v0
> would require independent protocol design (there being no analogous
> position to signal support for "unborn") and client-side plumbing of the
> data required, so the scope of this patch set is limited to protocol v2.
>
> The client side will be updated to use this in a subsequent commit.
>
> Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> ---
>  Documentation/config.txt                |  2 +
>  Documentation/config/lsrefs.txt         |  3 ++
>  Documentation/technical/protocol-v2.txt | 10 ++++-
>  ls-refs.c                               | 53 +++++++++++++++++++++++--
>  ls-refs.h                               |  1 +
>  serve.c                                 |  2 +-
>  t/t5701-git-serve.sh                    |  2 +-
>  7 files changed, 66 insertions(+), 7 deletions(-)
>  create mode 100644 Documentation/config/lsrefs.txt
>
> diff --git a/Documentation/config.txt b/Documentation/config.txt
> index 6ba50b1104..d08e83a148 100644
> --- a/Documentation/config.txt
> +++ b/Documentation/config.txt
> @@ -398,6 +398,8 @@ include::config/interactive.txt[]
>  
>  include::config/log.txt[]
>  
> +include::config/lsrefs.txt[]
> +
>  include::config/mailinfo.txt[]
>  
>  include::config/mailmap.txt[]
> diff --git a/Documentation/config/lsrefs.txt b/Documentation/config/lsrefs.txt
> new file mode 100644
> index 0000000000..dcbec11aaa
> --- /dev/null
> +++ b/Documentation/config/lsrefs.txt
> @@ -0,0 +1,3 @@
> +lsrefs.allowUnborn::
> +	Allow the server to send information about unborn symrefs during the
> +	protocol v2 ref advertisement.
> diff --git a/Documentation/technical/protocol-v2.txt b/Documentation/technical/protocol-v2.txt
> index 85daeb5d9e..4707511c10 100644
> --- a/Documentation/technical/protocol-v2.txt
> +++ b/Documentation/technical/protocol-v2.txt
> @@ -192,11 +192,19 @@ ls-refs takes in the following arguments:
>  	When specified, only references having a prefix matching one of
>  	the provided prefixes are displayed.
>  
> +If the 'unborn' feature is advertised the following argument can be
> +included in the client's request.
> +
> +    unborn
> +	The server may send symrefs pointing to unborn branches in the form
> +	"unborn <refname> symref-target:<target>".
> +
>  The output of ls-refs is as follows:
>  
>      output = *ref
>  	     flush-pkt
> -    ref = PKT-LINE(obj-id SP refname *(SP ref-attribute) LF)
> +    obj-id-or-unborn = (obj-id | "unborn")
> +    ref = PKT-LINE(obj-id-or-unborn SP refname *(SP ref-attribute) LF)
>      ref-attribute = (symref | peeled)
>      symref = "symref-target:" symref-target
>      peeled = "peeled:" obj-id
> diff --git a/ls-refs.c b/ls-refs.c
> index a1e0b473e4..4077adeb6a 100644
> --- a/ls-refs.c
> +++ b/ls-refs.c
> @@ -32,6 +32,8 @@ struct ls_refs_data {
>  	unsigned peel;
>  	unsigned symrefs;
>  	struct strvec prefixes;
> +	unsigned allow_unborn : 1;
> +	unsigned unborn : 1;
>  };
>  
>  static int send_ref(const char *refname, const struct object_id *oid,
> @@ -47,7 +49,10 @@ static int send_ref(const char *refname, const struct object_id *oid,
>  	if (!ref_match(&data->prefixes, refname_nons))
>  		return 0;
>  
> -	strbuf_addf(&refline, "%s %s", oid_to_hex(oid), refname_nons);
> +	if (oid)
> +		strbuf_addf(&refline, "%s %s", oid_to_hex(oid), refname_nons);
> +	else
> +		strbuf_addf(&refline, "unborn %s", refname_nons);

When a call is made to this helper with NULL "oid", it unconditionally sends the "refname" out as an 'unborn' thing. If data->symrefs is not true, or flag does not have REF_ISSYMREF set, then we'd end up sending

    "unborn" SP refname LF

without any ref-attribute. The caller is responsible for ensuring that it passes sensible data->symrefs and flag when it passes oid==NULL to this function, but it is OK because this is a private helper.

OK.
Show 22 quoted lines
>  	if (data->symrefs && flag & REF_ISSYMREF) {
>  		struct object_id unused;
>  		const char *symref_target = resolve_ref_unsafe(refname, 0,
> @@ -74,8 +79,30 @@ static int send_ref(const char *refname, const struct object_id *oid,
>  	return 0;
>  }
>  
> -static int ls_refs_config(const char *var, const char *value, void *data)
> +static void send_possibly_unborn_head(struct ls_refs_data *data)
>  {
> +	struct strbuf namespaced = STRBUF_INIT;
> +	struct object_id oid;
> +	int flag;
> +	int oid_is_null;
> +
> +	strbuf_addf(&namespaced, "%sHEAD", get_git_namespace());
> +	if (!resolve_ref_unsafe(namespaced.buf, 0, &oid, &flag))
> +		return; /* bad ref */
> +	oid_is_null = is_null_oid(&oid);
> +	if (!oid_is_null ||
> +	    (data->unborn && data->symrefs && (flag & REF_ISSYMREF)))
> +		send_ref(namespaced.buf, oid_is_null ? NULL : &oid, flag, data);
And this caller makes sure that send_ref()'s expectation holds.
Show 20 quoted lines
> +	strbuf_release(&namespaced);
> +}
> +
> +static int ls_refs_config(const char *var, const char *value, void *cb_data)
> +{
> +	struct ls_refs_data *data = cb_data;
> +
> +	if (!strcmp("lsrefs.allowunborn", var))
> +		data->allow_unborn = git_config_bool(var, value);
> +
>  	/*
>  	 * We only serve fetches over v2 for now, so respect only "uploadpack"
>  	 * config. This may need to eventually be expanded to "receive", but we
> @@ -91,7 +118,8 @@ int ls_refs(struct repository *r, struct strvec *keys,
>  
>  	memset(&data, 0, sizeof(data));
>  
> -	git_config(ls_refs_config, NULL);
> +	data.allow_unborn = 1;
> +	git_config(ls_refs_config, &data);

The above is a usual sequence of "an unspecified allow-unborn defaults to true, but the configuration can turn it off". OK

Show 9 quoted lines
>  
>  	while (packet_reader_read(request) == PACKET_READ_NORMAL) {
>  		const char *arg = request->line;
> @@ -103,14 +131,31 @@ int ls_refs(struct repository *r, struct strvec *keys,
>  			data.symrefs = 1;
>  		else if (skip_prefix(arg, "ref-prefix ", &out))
>  			strvec_push(&data.prefixes, out);
> +		else if (data.allow_unborn && !strcmp("unborn", arg))
> +			data.unborn = 1;

I think the use of &&-cascade is iffy here. Even when we are *not* accepting request for unborn, we should still parse it as such. This does not matter in today's code, but it is a basic courtesy for future developers who may add more "else if" after it.

IOW
		else if (!strcmp("unborn", arg)) {
			if (!data.allow_unborn)
				; /* we are not accepting the request */
			else
				data.unborn = 1;
		}

I wrote the above in longhand only for documentation purposes; in practice, 

		else if (!strcmp("unborn", arg))
                	data.unborn = data.allow_unborn;
may suffice.
Show 8 quoted lines
>  	}
>  
>  	if (request->status != PACKET_READ_FLUSH)
>  		die(_("expected flush after ls-refs arguments"));
>  
> -	head_ref_namespaced(send_ref, &data);
> +	send_possibly_unborn_head(&data);
>  	for_each_namespaced_ref(send_ref, &data);

And here is another caller of send_ref(). Are we sure that send_ref()'s expectation is satisfied by this caller when the iteration encounters a broken ref (e.g. refs/heads/broken not a symref but names an object that does not exist and get_sha1() yielding 0{40}), or a dangling symref (e.g. refs/remotes/origin/HEAD pointing at something that does not exist)?

Show 16 quoted lines
>  	packet_flush(1);
>  	strvec_clear(&data.prefixes);
>  	return 0;
>  }
> +
> +int ls_refs_advertise(struct repository *r, struct strbuf *value)
> +{
> +	if (value) {
> +		int allow_unborn_value;
> +
> +		if (repo_config_get_bool(the_repository,
> +					 "lsrefs.allowunborn",
> +					 &allow_unborn_value) ||
> +		    allow_unborn_value)
> +			strbuf_addstr(value, "unborn");
> +	}
This reads "when not explicitly disabled, stuff "unborn" in there".

It feels somewhat brittle that we have to read the same variable and apply the same "default to true" logic in two places and have to keep them in sync. Is this because the decision to advertize or not has to be made way before the code that is specific to the implementation of ls-refs is run?

If ls_refs_advertise() is always called first before ls_refs(), I wonder if it makes sense to reuse what we found out about the configured (or left unconfigured) state here and use it when ls_refs() gets called? I know that the way serve.c infrastructure calls "do we advertise?" helper from each protocol-element handler is too narrow and does not allow us to pass such a necessary piece of information but I view it as a misdesign that can be corrected (and until that happens, we could use file-local static limited to ls-refs.c).

Show 39 quoted lines
> +	return 1;
> +}
> diff --git a/ls-refs.h b/ls-refs.h
> index 7b33a7c6b8..a99e4be0bd 100644
> --- a/ls-refs.h
> +++ b/ls-refs.h
> @@ -6,5 +6,6 @@ struct strvec;
>  struct packet_reader;
>  int ls_refs(struct repository *r, struct strvec *keys,
>  	    struct packet_reader *request);
> +int ls_refs_advertise(struct repository *r, struct strbuf *value);
>  
>  #endif /* LS_REFS_H */
> diff --git a/serve.c b/serve.c
> index eec2fe6f29..ac20c72763 100644
> --- a/serve.c
> +++ b/serve.c
> @@ -73,7 +73,7 @@ struct protocol_capability {
>  
>  static struct protocol_capability capabilities[] = {
>  	{ "agent", agent_advertise, NULL },
> -	{ "ls-refs", always_advertise, ls_refs },
> +	{ "ls-refs", ls_refs_advertise, ls_refs },
>  	{ "fetch", upload_pack_advertise, upload_pack_v2 },
>  	{ "server-option", always_advertise, NULL },
>  	{ "object-format", object_format_advertise, NULL },
> diff --git a/t/t5701-git-serve.sh b/t/t5701-git-serve.sh
> index a1f5fdc9fd..df29504161 100755
> --- a/t/t5701-git-serve.sh
> +++ b/t/t5701-git-serve.sh
> @@ -12,7 +12,7 @@ test_expect_success 'test capability advertisement' '
>  	cat >expect <<-EOF &&
>  	version 2
>  	agent=git/$(git version | cut -d" " -f3)
> -	ls-refs
> +	ls-refs=unborn
>  	fetch=shallow
>  	server-option
>  	object-format=$(test_oid algo)
Previous: Jonathan TanNext: Jeff King
Message 61 of 109 in “Cloning empty repository uses locally configured default branch name”
  1. Jonathan TanDec 8, 2020
  2. Junio C HamanoDec 8, 2020
  3. brian m. carlsonDec 8, 2020
  4. Jonathan TanDec 8, 2020
  5. Junio C HamanoDec 8, 2020
  6. Jeff KingDec 8, 2020
  7. Jonathan TanDec 8, 2020
  8. Jeff KingDec 8, 2020
  9. clone: in protocol v2, use remote's default branchJonathan Tan, Dec 11, 2020
  10. Junio C HamanoDec 11, 2020
  11. Ævar Arnfjörð BjarmasonDec 14, 2020
  12. Felipe ContrerasDec 14, 2020
  13. Junio C HamanoDec 14, 2020
  14. Ævar Arnfjörð BjarmasonDec 15, 2020
  15. Junio C HamanoDec 15, 2020
  16. Jeff KingDec 15, 2020
  17. Junio C HamanoDec 15, 2020
  18. Jeff KingDec 15, 2020
  19. Junio C HamanoDec 16, 2020
  20. Jeff KingDec 16, 2020
  21. Junio C HamanoDec 16, 2020
  22. Jeff KingDec 18, 2020
  23. Felipe ContrerasDec 15, 2020
  24. Jonathan TanDec 14, 2020
  25. Felipe ContrerasDec 14, 2020
  26. Jeff KingDec 15, 2020
  27. Jonathan TanDec 15, 2020
  28. 0/3 Cloning with remote unborn HEADJonathan Tan, Dec 16, 2020
  29. 1/3 ls-refs: report unborn targets of symrefsJonathan Tan, Dec 16, 2020
  30. Junio C HamanoDec 16, 2020
  31. Jonathan TanDec 16, 2020
  32. Jeff KingDec 16, 2020
  33. Jonathan TanDec 16, 2020
  34. Junio C HamanoDec 17, 2020
  35. Jeff KingDec 18, 2020
  36. 2/3 connect, transport: add no-op arg for future patchJonathan Tan, Dec 16, 2020
  37. Junio C HamanoDec 16, 2020
  38. 3/3 clone: respect remote unborn HEADJonathan Tan, Dec 16, 2020
  39. 0/3 Cloning with remote unborn HEADJonathan Tan, Dec 21, 2020
  40. 1/3 ls-refs: report unborn targets of symrefsJonathan Tan, Dec 21, 2020
  41. 2/3 connect, transport: add no-op arg for future patchJonathan Tan, Dec 21, 2020
  42. 3/3 clone: respect remote unborn HEADJonathan Tan, Dec 21, 2020
  43. Junio C HamanoDec 21, 2020
  44. Jeff KingJan 21, 2021
  45. 0/3 Cloning with remote unborn HEADJonathan Tan, Dec 22, 2020
  46. 1/3 ls-refs: report unborn targets of symrefsJonathan Tan, Dec 22, 2020
  47. Jeff KingJan 21, 2021
  48. Jonathan TanJan 26, 2021
  49. Jeff KingJan 26, 2021
  50. 2/3 connect, transport: add no-op arg for future patchJonathan Tan, Dec 22, 2020
  51. Jeff KingJan 21, 2021
  52. Jonathan TanJan 26, 2021
  53. 3/3 clone: respect remote unborn HEADJonathan Tan, Dec 22, 2020
  54. Jeff KingJan 21, 2021
  55. Jonathan TanJan 26, 2021
  56. Jeff KingJan 26, 2021
  57. Junio C HamanoJan 28, 2021
  58. Junio C HamanoDec 22, 2020
  59. 0/3 Cloning with remote unborn HEADJonathan Tan, Jan 26, 2021
  60. 1/3 ls-refs: report unborn targets of symrefsJonathan Tan, Jan 26, 2021
  61. Junio C HamanoJan 26, 2021
  62. Jeff KingJan 26, 2021
  63. Junio C HamanoJan 26, 2021
  64. Junio C HamanoJan 26, 2021
  65. Jonathan TanJan 30, 2021
  66. Jonathan TanJan 29, 2021
  67. Junio C HamanoJan 29, 2021
  68. Jonathan TanFeb 2, 2021
  69. Junio C HamanoFeb 2, 2021
  70. Ævar Arnfjörð BjarmasonJan 27, 2021
  71. Jonathan TanJan 30, 2021
  72. 2/3 connect, transport: encapsulate arg in structJonathan Tan, Jan 26, 2021
  73. Junio C HamanoJan 26, 2021
  74. Jonathan TanJan 30, 2021
  75. 3/3 clone: respect remote unborn HEADJonathan Tan, Jan 26, 2021
  76. Junio C HamanoJan 26, 2021
  77. Jonathan TanJan 30, 2021
  78. Ævar Arnfjörð BjarmasonJan 27, 2021
  79. Jonathan TanJan 30, 2021
  80. Ævar Arnfjörð BjarmasonJan 30, 2021
  81. Jonathan TanFeb 2, 2021
  82. Ævar Arnfjörð BjarmasonFeb 3, 2021
  83. Junio C HamanoFeb 5, 2021
  84. Junio C HamanoJan 27, 2021
  85. Jeff KingJan 27, 2021
  86. Junio C HamanoJan 27, 2021
  87. 0/3 Cloning with remote unborn HEADJonathan Tan, Feb 2, 2021
  88. 1/3 ls-refs: report unborn targets of symrefsJonathan Tan, Feb 2, 2021
  89. Junio C HamanoFeb 2, 2021
  90. Jonathan TanFeb 2, 2021
  91. Junio C HamanoFeb 2, 2021
  92. Jonathan TanFeb 3, 2021
  93. 2/3 connect, transport: encapsulate arg in structJonathan Tan, Feb 2, 2021
  94. 3/3 clone: respect remote unborn HEADJonathan Tan, Feb 2, 2021
  95. 0/3 Cloning with remote unborn HEADJonathan Tan, Feb 5, 2021
  96. 1/3 ls-refs: report unborn targets of symrefsJonathan Tan, Feb 5, 2021
  97. Jeff KingFeb 5, 2021
  98. 2/3 connect, transport: encapsulate arg in structJonathan Tan, Feb 5, 2021
  99. 3/3 clone: respect remote unborn HEADJonathan Tan, Feb 5, 2021
  100. Junio C HamanoFeb 5, 2021
  101. Ævar Arnfjörð BjarmasonFeb 5, 2021
  102. Junio C HamanoFeb 5, 2021
  103. Jeff KingFeb 5, 2021
  104. 0/3 Cloning with remote unborn HEADJonathan Tan, Feb 5, 2021
  105. 3/3 clone: respect remote unborn HEADJonathan Tan, Feb 5, 2021
  106. 2/3 connect, transport: encapsulate arg in structJonathan Tan, Feb 5, 2021
  107. 1/3 ls-refs: report unborn targets of symrefsJonathan Tan, Feb 5, 2021
  108. Junio C HamanoFeb 6, 2021
  109. Junio C HamanoFeb 8, 2021

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