Backing up git repos on multiple machines to central repo w/o collision

Git Tree Management

Here's a quick column about git. It's not a complete how-to or tutorial, but more an interesting way to manage multiple trees.

The problem: I have a dozen trees on a half dozen machines. I'd like at least backup all the branches in these trees to github. Trouble is, I don't want branch names to step on each other. This can happen for a number of reasons, let's say I called something 'junk' by habit on N trees and don't want a push to screw that up...

Git's world view: To understand git, you have to understand that it is a graph of versioned trees with labels. Each node in the tree has the familiar hash, and some of the hashes have refs that the git branch command groks. It's all just a directed graph with labels under the covers.

Normally, you when clone a repo, all its tags magically change from foo to origin/foo (for some value of origin).

Enter refspecs

Turns out this has been thought of before. The answer is simple:

fred% git push origin foo:fred/foo

will push the foo branch to your origin and rewrite its name to fred/foo. Don't forget to push master too.

Now when you go to barney and fetch, you'll have a bunch of remote branches named origin/fred/foo, etc.

Costs

Since I'm doing this with a number of git svn trees, the cost is kinda high since git svn creates new, unique git hashes for all the upstream revisions to git svn rebase. It also means that you'll need to learn how to use the --onto arg of git-rebase, since if you want to move a branch from one repo like this to another.

barney% git checkout -b foo fred/foo
barney% git rebase -i fred/master foo --onto master

since you're effectively creating a new name space on your local machine for the new branch. The rebase will now properly take just those commits from foo, and then play them back onto master on the current machine and leave you with a 'foo' branch for the results.

Most things in the VENIX emulator are working

SUCCESS

So I got tired of the terrible progress I was making chasing down issues. I thought if I could just create a simple program and get that working, I'd have much better luck.

So I wrote a simple K&R style C program:

int a=123;
int b;
extern char *etext, *edata, *end;
main() {
int c;
printf("CS: %x DS: %x ES: %x SS: %x\n", getcs(), getds(), getes(), getss());
printf("&data = %x &bss = %x &stack = %x\n", &a, &b, &c);
printf("etext = %x edata = %x end = %x\n", &etext, &edata, &end);
}

which printed the segment addresses and then locations of the segments.

and I ran it on my old Rainbow running Venix.

I made some interesting discoveries. First, that there are two kinds of stacks (low and high) in addition to there being two kinds of binary (OMAGIC and NMAGIC). So my loader was all wrong. Next, I discovered I needed to jump to a_entry, not 0, to make low stack binaries work (all the ones I'd been testing so far were low stack, but somehow mostly worked when the stack and text segments were swapped).

Armed with this knowledge, I built 4 binaries (no flags, -z, -i, -i -z) to test all 4 cases. The -z ones worked (yea!) while the non-z ones didn't. My loader was right in this case, but I was returning EFAULT for all the writes. Why? Because I had a check in there to make sure the address was between 0 and brk. High stack binaries also have a valid area from sp() to 0xffff. When I added that change, all 4 test programs worked.

Of course, getting them from the Rainbow to the server was a challenge. The key to remember here is that you needed to use 'set line /dev/com1.m' on the rainbow so that kermit would on login port. I also had to down-clock to 2400 baud to get it reliable.

So, I started testing a lot of programs that failed to work before. Sort(1) is now working. ls isn't, but comes closer (it tries really hard to interpret a modern FreeBSD dirent as a v7 one and that's not so good, but that's fixable). nm is still giving me problems, for reasons unknown. I have enough things working, though, that I can start to try out as, ld and friends. Maybe even cc (though I'd need to get both fork and signals working for that driver program). /bin/sh fails missing dup() (and likely a bunch of others).

So excellent progress in the last few days.

Even more VENIX emulator progress

So in looking at the traces for why cal wasn't working, I noticed something odd:

0212:100D: jmp 0x109e
0212:109C: rcrw $0xff,0xdceb(%bx,%si)
Invalid opcode c1

What? I'm not super-duper strong on Intel assembler, but I sure know that 109e is not 109c. So what's going on here. After adding some more debugging, I discovered this was opcode 0xe9, which is a jump relative with word (so take IP and add the next two bytes to it). So, the code looked OK:

                doJump(ip + fetchWord()); 

But looking more closely. It's oddly off by 2. Sow what's inside fetchWord()? Inside it effectively does ip++, twice. So, on the other compiler that was used for this code that I obtained from tkchia's reenigne repo had this flaw. it did fetchWord() + ip, rather than clang's ip + fetchWord(). So the fix was simple:

                t = fetchWord();
                doJump(ip + t);

which made the order of operations well defined. A quick audit of the code shows no other places where this is done.

I did that, and nothing else, and now cal(1) works. It produces correct calendars. As does od(1), uniq(1), pr(1) and others.

I've also implemented alarm(2), signal(2), lseek(2) and pause(2). With that, sleep(1) works (although it says 'Alarm clock' which suggests I need to actually establish a SIGALRM handler).

There's enough working I'm starting to need some kind of regression suite to make sure I don't regress and can publish the status of all the binaries... Maybe I could leverage an existing something...

VENIX/86 emulator taking shape...

Frequent readers will recall my obsession with Venix on the Rainbow.

For the past year or so in my off moments, I've been trying to put together a Venix binary emulator. This is part of a larger project to reconstruct the Venix sources from the ancient V7 sources plus clues left behind in various images found on the internet in time for the 50th anniversary of Unix next year.

Early in the summer I had the loader written. I could successfully load a VENIX image and start it executing. sync(1) was working, but it did little more than call the sync(2) and _exit(2) system calls. stdio programs were still basically not working, though odd things like ln(1) also worked.

My project is now one step closer to fruition. I have been able to get some of the basic programs in /bin working with my emulator. The last step before getting to this point was finding a bug in the 8086 CPU emulation where the pointers to things like AH and AL for the AX register were wrong so that "movb al, 1" would set ah to 1... To find that I ported ddb from FreeBSD over so I could print out registers and disassembled code that's about to execute and pore over the changes to the registers until I could spot a problem...

With that fixed, all the super simple programs run. echo(1), cat(1), tr(1), and basename(1) run. However, other simple ones like rm(1), touch(1), ls(1), and wc(1) all run into problems.

touch(1) seems to be related to a bad implementation of stat(2), for example, that I've not had the time to chase down. mv(1) and rm(1) seem that way as well. No clue what's up with wc(1) or ls(1).

I think I should come up with some kind of test script(s) to make sure the basics work.

Venix Github repo has the 86sim program in it.

I hope to soon be to the point where everything except maybe fork/exec works. I'll need those for not only /bin/sh, but also cc. cc(1) is the reason that I want to make this work so I can rebuild everything quickly for the VENIX restoration project....