Re: git fsck: unreachable vs. dangling
Junio C Hamano venit, vidit, dixit 14.04.2015 11:22: > Sebastian Schuberth writes: > >> On Tue, Apr 14, 2015 at 10:50 AM, Michael J Gruber >> wrote: >> >>> "to dangle" means "to hang loosely". >>> >>> So, in the description above, "A^ dangles from A loosely" because it >>> hangs from A (you can reach it from A) but loosely, because it would >>> "drop" if A gets dropped and A is "likely" to be dropped (because it is >>> unreachable by refs). But A^ is not dangling in our terminology. >>> >>> If you *reverse the arrows*, i.e. consider A^ pointing to A, it becomes >>> more apparent that A is dangling: it is an unreferenced leaf node. >> >> That's exactly what confused me. In the very literal sense, something >> can only "hang loosely", i.e. dangle, if it's only tied at *one* end, >> and that's the case for A (which is only connected to A^) but not for >> A^ (which is connected to its parent, and A). Especially when talking >> about A as a "leaf" node, like in the leaf of a natural tree, I would >> think that A is dangling. > > I am not sure if I follow, but probably it is just me who is not > strong at math, or whose eyesight is not keen enough to notice the > arrow heads on links between the commits. "git log --graph" does not show arrow heads, obviously. Many illustrations about Git do. The relation between commits is clearly directed: A being a parent commit of B is different from B being a parent commit of A (and both cannot be true simultaneously due to the "A" in "DAG") > I just visualize commits to be ping-pong balls with strings between > them, and then grab the root of the graph and lift the whole thing > up, while tips of the branches and tags are anchored. Commit A will > be dangling in the wind if you shake the whole thing. If you don't have a concept of direction it is difficult to distinguish roots from tips... Our commit relationship is certainly a directed one. You can define it using either "is parent of" or "is child of". They are opposite, and lead to opposite notions of "root" (a node without predecessors) and "tip" (a node without successors). > But that visualization breaks down once you start thinking about > what will happen to A^{tree} and its blobs; they are attached to A > with thin strings and they will have to float above A (i.e. sit > somewhere closer to the root of the tree) just like A^ will go > closer to the root, to make A appear the "dangling" one, as the > direction of the arrow is from A to A^{tree} just like we have an > arrow from A to A^; just like A^ is not dangling because of A, > A^{tree} is not dangling. > -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: git fsck: unreachable vs. dangling
On Tue, Apr 14, 2015 at 10:52 AM, Sebastian Schuberth wrote: >> A dangling object is an unreachable object that cannot be >> made reachable by any way other than pointing at it >> directly with a ref. > > Thanks a lot for the prompt explanation! Note to myself: I just realized that both "dangling" and "unreachable" are also nicely defined in the Git glossary [1]. [1] http://git-scm.com/docs/gitglossary/ -- Sebastian Schuberth -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: git fsck: unreachable vs. dangling
On Tue, Apr 14, 2015 at 11:22 AM, Junio C Hamano wrote: > I just visualize commits to be ping-pong balls with strings between > them, and then grab the root of the graph and lift the whole thing > up, while tips of the branches and tags are anchored. Commit A will > be dangling in the wind if you shake the whole thing. I used to have exactly the same visualization in mind, but got confused in between, unsure whether my understanding was correct. As it turns out it is, and when sticking to that visualization everything seems to be consistent in the fsck docs. They could still benefit some more clarification I guess. I'll see if I can come up with something. Thanks again. -- Sebastian Schuberth -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: git fsck: unreachable vs. dangling
Sebastian Schuberth writes: > On Tue, Apr 14, 2015 at 10:50 AM, Michael J Gruber > wrote: > >> "to dangle" means "to hang loosely". >> >> So, in the description above, "A^ dangles from A loosely" because it >> hangs from A (you can reach it from A) but loosely, because it would >> "drop" if A gets dropped and A is "likely" to be dropped (because it is >> unreachable by refs). But A^ is not dangling in our terminology. >> >> If you *reverse the arrows*, i.e. consider A^ pointing to A, it becomes >> more apparent that A is dangling: it is an unreferenced leaf node. > > That's exactly what confused me. In the very literal sense, something > can only "hang loosely", i.e. dangle, if it's only tied at *one* end, > and that's the case for A (which is only connected to A^) but not for > A^ (which is connected to its parent, and A). Especially when talking > about A as a "leaf" node, like in the leaf of a natural tree, I would > think that A is dangling. I am not sure if I follow, but probably it is just me who is not strong at math, or whose eyesight is not keen enough to notice the arrow heads on links between the commits. I just visualize commits to be ping-pong balls with strings between them, and then grab the root of the graph and lift the whole thing up, while tips of the branches and tags are anchored. Commit A will be dangling in the wind if you shake the whole thing. But that visualization breaks down once you start thinking about what will happen to A^{tree} and its blobs; they are attached to A with thin strings and they will have to float above A (i.e. sit somewhere closer to the root of the tree) just like A^ will go closer to the root, to make A appear the "dangling" one, as the direction of the arrow is from A to A^{tree} just like we have an arrow from A to A^; just like A^ is not dangling because of A, A^{tree} is not dangling. -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: git fsck: unreachable vs. dangling
On Tue, Apr 14, 2015 at 10:50 AM, Michael J Gruber wrote: > "to dangle" means "to hang loosely". > > So, in the description above, "A^ dangles from A loosely" because it > hangs from A (you can reach it from A) but loosely, because it would > "drop" if A gets dropped and A is "likely" to be dropped (because it is > unreachable by refs). But A^ is not dangling in our terminology. > > If you *reverse the arrows*, i.e. consider A^ pointing to A, it becomes > more apparent that A is dangling: it is an unreferenced leaf node. That's exactly what confused me. In the very literal sense, something can only "hang loosely", i.e. dangle, if it's only tied at *one* end, and that's the case for A (which is only connected to A^) but not for A^ (which is connected to its parent, and A). Especially when talking about A as a "leaf" node, like in the leaf of a natural tree, I would think that A is dangling. -- Sebastian Schuberth -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: git fsck: unreachable vs. dangling
On Tue, Apr 14, 2015 at 10:05 AM, Junio C Hamano wrote: > A dangling object is an unreachable object that cannot be > made reachable by any way other than pointing at it > directly with a ref. Thanks a lot for the prompt explanation! -- Sebastian Schuberth -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: git fsck: unreachable vs. dangling
Junio C Hamano venit, vidit, dixit 14.04.2015 10:05: > On Tue, Apr 14, 2015 at 12:16 AM, Sebastian Schuberth > wrote: >> Hi, >> >> reading through the fsck docs [1] I'm having a hard time understanding >> what the difference between "unreachable" and "dangling" objects are. >> >> By example, suppose I have a commit A that is the tip of exactly one >> branch (and no tag or other ref points to A). If I delete that branch, >> is A now dangling, or unreachable, or both? > > Suppose that branch consists of two commits, A and A^. > When you lose that branch (git branch -D that-branch), > both A and A^ become unreachable. So are trees and > blobs that appear only in A and A^ and nowhere else; > they are also unreachable. > > A dangling object is an unreachable object that cannot be > made reachable by any way other than pointing at it > directly with a ref. A^ is not dangling, because you can > make it reachable by pointing A (the tip of the original > branch you just lost) with a ref. A on the other hand is > dangling (if you had a tag object that points at A that > you lost, then A is merely unreachable but not dangling, > because you can point at that tag with a ref and make A > reachable). > This terminology is established, but misleading if you try to understanding things from the word and the technical tree structure: "to dangle" means "to hang loosely". So, in the description above, "A^ dangles from A loosely" because it hangs from A (you can reach it from A) but loosely, because it would "drop" if A gets dropped and A is "likely" to be dropped (because it is unreachable by refs). But A^ is not dangling in our terminology. If you *reverse the arrows*, i.e. consider A^ pointing to A, it becomes more apparent that A is dangling: it is an unreferenced leaf node. But really, we're switching directions of the arrows again and again when, on the one hand, we talk about refs pointing to commits, commits pointing to parent commits (which they are, of course) but then, on the other hand, use terms like "root" and "dangling" which make sense only when you think of the oppositely oriented tree. Maybe we should introduce the terms DAG and reverse DAG (GAD?) to make this clearer. Our DAG is technically oriented from commits to their parents all the way "up" to the root. Visually, we often orient it from the root towards child commits. When we do a revision walk, do we walk forward or backwards? I'd say users think in the ordering in which they create commits (from root to children), let's call that DAG. It explains "root" and "dangle" and such, and a revision walk is a walk backwards in that orientation. Technically, our acyclic graph is oriented the other way, a revision walk walks forward in that orientation - it couldn't walk any other way without extensive searching. This orientation (GAD?) explains "points to", be it "ref to rev" or "commit to parent". Michael P.S.: The more mathematically inclined will notice that we can have more than 2 orientations if our graph is not connected... But I'd say we have one "technical" orientation (commit->parent) and the opposite and can forget about others. -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: git fsck: unreachable vs. dangling
On Tue, Apr 14, 2015 at 12:16 AM, Sebastian Schuberth wrote: > Hi, > > reading through the fsck docs [1] I'm having a hard time understanding > what the difference between "unreachable" and "dangling" objects are. > > By example, suppose I have a commit A that is the tip of exactly one > branch (and no tag or other ref points to A). If I delete that branch, > is A now dangling, or unreachable, or both? Suppose that branch consists of two commits, A and A^. When you lose that branch (git branch -D that-branch), both A and A^ become unreachable. So are trees and blobs that appear only in A and A^ and nowhere else; they are also unreachable. A dangling object is an unreachable object that cannot be made reachable by any way other than pointing at it directly with a ref. A^ is not dangling, because you can make it reachable by pointing A (the tip of the original branch you just lost) with a ref. A on the other hand is dangling (if you had a tag object that points at A that you lost, then A is merely unreachable but not dangling, because you can point at that tag with a ref and make A reachable). -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html