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diff --git a/Documentation/technical/api-credentials.txt b/Documentation/technical/api-credentials.txt new file mode 100644 index 0000000000..e44426dd04 --- /dev/null +++ b/Documentation/technical/api-credentials.txt @@ -0,0 +1,271 @@ +credentials API +=============== + +The credentials API provides an abstracted way of gathering username and +password credentials from the user (even though credentials in the wider +world can take many forms, in this document the word "credential" always +refers to a username and password pair). + +This document describes two interfaces: the C API that the credential +subsystem provides to the rest of Git, and the protocol that Git uses to +communicate with system-specific "credential helpers". If you are +writing Git code that wants to look up or prompt for credentials, see +the section "C API" below. If you want to write your own helper, see +the section on "Credential Helpers" below. + +Typical setup +------------- + +------------ ++-----------------------+ +| Git code (C) |--- to server requiring ---> +| | authentication +|.......................| +| C credential API |--- prompt ---> User ++-----------------------+ + ^ | + | pipe | + | v ++-----------------------+ +| Git credential helper | ++-----------------------+ +------------ + +The Git code (typically a remote-helper) will call the C API to obtain +credential data like a login/password pair (credential_fill). The +API will itself call a remote helper (e.g. "git credential-cache" or +"git credential-store") that may retrieve credential data from a +store. If the credential helper cannot find the information, the C API +will prompt the user. Then, the caller of the API takes care of +contacting the server, and does the actual authentication. + +C API +----- + +The credential C API is meant to be called by Git code which needs to +acquire or store a credential. It is centered around an object +representing a single credential and provides three basic operations: +fill (acquire credentials by calling helpers and/or prompting the user), +approve (mark a credential as successfully used so that it can be stored +for later use), and reject (mark a credential as unsuccessful so that it +can be erased from any persistent storage). + +Data Structures +~~~~~~~~~~~~~~~ + +`struct credential`:: + + This struct represents a single username/password combination + along with any associated context. All string fields should be + heap-allocated (or NULL if they are not known or not applicable). + The meaning of the individual context fields is the same as + their counterparts in the helper protocol; see the section below + for a description of each field. ++ +The `helpers` member of the struct is a `string_list` of helpers. Each +string specifies an external helper which will be run, in order, to +either acquire or store credentials. See the section on credential +helpers below. This list is filled-in by the API functions +according to the corresponding configuration variables before +consulting helpers, so there usually is no need for a caller to +modify the helpers field at all. ++ +This struct should always be initialized with `CREDENTIAL_INIT` or +`credential_init`. + + +Functions +~~~~~~~~~ + +`credential_init`:: + + Initialize a credential structure, setting all fields to empty. + +`credential_clear`:: + + Free any resources associated with the credential structure, + returning it to a pristine initialized state. + +`credential_fill`:: + + Instruct the credential subsystem to fill the username and + password fields of the passed credential struct by first + consulting helpers, then asking the user. After this function + returns, the username and password fields of the credential are + guaranteed to be non-NULL. If an error occurs, the function will + die(). + +`credential_reject`:: + + Inform the credential subsystem that the provided credentials + have been rejected. This will cause the credential subsystem to + notify any helpers of the rejection (which allows them, for + example, to purge the invalid credentials from storage). It + will also free() the username and password fields of the + credential and set them to NULL (readying the credential for + another call to `credential_fill`). Any errors from helpers are + ignored. + +`credential_approve`:: + + Inform the credential subsystem that the provided credentials + were successfully used for authentication. This will cause the + credential subsystem to notify any helpers of the approval, so + that they may store the result to be used again. Any errors + from helpers are ignored. + +`credential_from_url`:: + + Parse a URL into broken-down credential fields. + +Example +~~~~~~~ + +The example below shows how the functions of the credential API could be +used to login to a fictitious "foo" service on a remote host: + +----------------------------------------------------------------------- +int foo_login(struct foo_connection *f) +{ + int status; + /* + * Create a credential with some context; we don't yet know the + * username or password. + */ + + struct credential c = CREDENTIAL_INIT; + c.protocol = xstrdup("foo"); + c.host = xstrdup(f->hostname); + + /* + * Fill in the username and password fields by contacting + * helpers and/or asking the user. The function will die if it + * fails. + */ + credential_fill(&c); + + /* + * Otherwise, we have a username and password. Try to use it. + */ + status = send_foo_login(f, c.username, c.password); + switch (status) { + case FOO_OK: + /* It worked. Store the credential for later use. */ + credential_accept(&c); + break; + case FOO_BAD_LOGIN: + /* Erase the credential from storage so we don't try it + * again. */ + credential_reject(&c); + break; + default: + /* + * Some other error occurred. We don't know if the + * credential is good or bad, so report nothing to the + * credential subsystem. + */ + } + + /* Free any associated resources. */ + credential_clear(&c); + + return status; +} +----------------------------------------------------------------------- + + +Credential Helpers +------------------ + +Credential helpers are programs executed by Git to fetch or save +credentials from and to long-term storage (where "long-term" is simply +longer than a single Git process; e.g., credentials may be stored +in-memory for a few minutes, or indefinitely on disk). + +Each helper is specified by a single string in the configuration +variable `credential.helper` (and others, see linkgit:git-config[1]). +The string is transformed by Git into a command to be executed using +these rules: + + 1. If the helper string begins with "!", it is considered a shell + snippet, and everything after the "!" becomes the command. + + 2. Otherwise, if the helper string begins with an absolute path, the + verbatim helper string becomes the command. + + 3. Otherwise, the string "git credential-" is prepended to the helper + string, and the result becomes the command. + +The resulting command then has an "operation" argument appended to it +(see below for details), and the result is executed by the shell. + +Here are some example specifications: + +---------------------------------------------------- +# run "git credential-foo" +foo + +# same as above, but pass an argument to the helper +foo --bar=baz + +# the arguments are parsed by the shell, so use shell +# quoting if necessary +foo --bar="whitespace arg" + +# you can also use an absolute path, which will not use the git wrapper +/path/to/my/helper --with-arguments + +# or you can specify your own shell snippet +!f() { echo "password=`cat $HOME/.secret`"; }; f +---------------------------------------------------- + +Generally speaking, rule (3) above is the simplest for users to specify. +Authors of credential helpers should make an effort to assist their +users by naming their program "git-credential-$NAME", and putting it in +the $PATH or $GIT_EXEC_PATH during installation, which will allow a user +to enable it with `git config credential.helper $NAME`. + +When a helper is executed, it will have one "operation" argument +appended to its command line, which is one of: + +`get`:: + + Return a matching credential, if any exists. + +`store`:: + + Store the credential, if applicable to the helper. + +`erase`:: + + Remove a matching credential, if any, from the helper's storage. + +The details of the credential will be provided on the helper's stdin +stream. The exact format is the same as the input/output format of the +`git credential` plumbing command (see the section `INPUT/OUTPUT +FORMAT` in linkgit:git-credential[7] for a detailed specification). + +For a `get` operation, the helper should produce a list of attributes +on stdout in the same format. A helper is free to produce a subset, or +even no values at all if it has nothing useful to provide. Any provided +attributes will overwrite those already known about by Git. If a helper +outputs a `quit` attribute with a value of `true` or `1`, no further +helpers will be consulted, nor will the user be prompted (if no +credential has been provided, the operation will then fail). + +For a `store` or `erase` operation, the helper's output is ignored. +If it fails to perform the requested operation, it may complain to +stderr to inform the user. If it does not support the requested +operation (e.g., a read-only store), it should silently ignore the +request. + +If a helper receives any other operation, it should silently ignore the +request. This leaves room for future operations to be added (older +helpers will just ignore the new requests). + +See also +-------- + +linkgit:gitcredentials[7] + +linkgit:git-config[5] (See configuration variables `credential.*`) |