merge(3p/git): Merge git subtree at v2.29.2 r/1890

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

1 files changed, 45 insertions, 0 deletions

diff --git a/third_party/git/sigchain.h b/third_party/git/sigchain.h
index 138b20f54b40..8e6bada8928c 100644
--- a/third_party/git/sigchain.h
+++ b/third_party/git/sigchain.h
@@ -1,12 +1,57 @@
 #ifndef SIGCHAIN_H
 #define SIGCHAIN_H
 
+/**
+ * Code often wants to set a signal handler to clean up temporary files or
+ * other work-in-progress when we die unexpectedly. For multiple pieces of
+ * code to do this without conflicting, each piece of code must remember
+ * the old value of the handler and restore it either when:
+ *
+ *   1. The work-in-progress is finished, and the handler is no longer
+ *      necessary. The handler should revert to the original behavior
+ *      (either another handler, SIG_DFL, or SIG_IGN).
+ *
+ *   2. The signal is received. We should then do our cleanup, then chain
+ *      to the next handler (or die if it is SIG_DFL).
+ *
+ * Sigchain is a tiny library for keeping a stack of handlers. Your handler
+ * and installation code should look something like:
+ *
+ * ------------------------------------------
+ *   void clean_foo_on_signal(int sig)
+ *   {
+ * 	  clean_foo();
+ * 	  sigchain_pop(sig);
+ * 	  raise(sig);
+ *   }
+ *
+ *   void other_func()
+ *   {
+ * 	  sigchain_push_common(clean_foo_on_signal);
+ * 	  mess_up_foo();
+ * 	  clean_foo();
+ *   }
+ * ------------------------------------------
+ *
+ */
+
+/**
+ * Handlers are given the typedef of sigchain_fun. This is the same type
+ * that is given to signal() or sigaction(). It is perfectly reasonable to
+ * push SIG_DFL or SIG_IGN onto the stack.
+ */
 typedef void (*sigchain_fun)(int);
 
+/* You can sigchain_push and sigchain_pop individual signals. */
 int sigchain_push(int sig, sigchain_fun f);
 int sigchain_pop(int sig);
 
+/**
+ * push the handler onto the stack for the common signals:
+ * SIGINT, SIGHUP, SIGTERM, SIGQUIT and SIGPIPE.
+ */
 void sigchain_push_common(sigchain_fun f);
+
 void sigchain_pop_common(void);
 
 #endif /* SIGCHAIN_H */