Append data and becomes bigger than block_size/2

[palainp]

Hi, not sure how to translate that into the dune tests but I ran into an issue with the following C code when writing onto a chamelon fs:

#include <stdio.h>
#include <assert.h>
#include <string.h>

#define BLOC_SIZE 512
#define FN "test.bin"

char small[BLOC_SIZE/4-1];
char addi[BLOC_SIZE];

int main()
{
	FILE* fd;

	for (size_t i=0; i<sizeof(small); ++i)
		small[i]='#';

	for (size_t i=0; i<sizeof(addi); ++i)
		addi[i]='@';

	// create a "just a bit under quarter block_size" file
	assert((fd = fopen(FN, "r+")) != NULL);
	assert(fwrite(small, sizeof(char), sizeof(small), fd) == sizeof(small));
	assert(fflush(fd) == 0);
	assert(fclose(fd) == 0);

	// now append data at the end of FN
	assert((fd = fopen(FN, "r+")) != NULL);
	assert(fseek(fd, sizeof(small), SEEK_SET)!=-1);
	assert(fwrite(addi, sizeof(char), sizeof(addi), fd) == sizeof(addi));
	assert(fflush(fd) == 0);
	assert(fclose(fd) == 0);

	return 0;
}

This is my bumb function for set_partial, this is use for both fwrite calls (with offset arguments at 0, and block_size/4 respectively):

  let write root path ~offset newdata_length newdata =
    let pathkey = Mirage_kv.Key.v path in
    get_before root pathkey offset >>= fun before ->
    Log.warn (fun f -> f "got %d bytes before offset %d" (Cstruct.length before) offset) ;
    get_after root pathkey (offset+newdata_length) >>= fun after ->
    Log.warn (fun f -> f "got %d bytes after offset %d" (Cstruct.length after) (offset+newdata_length)) ;

    let newdata = Cstruct.concat [before; newdata; after] in
    Log.warn (fun f -> f "will write %d bytes" (Cstruct.length newdata)) ;
    Chamelon.remove root pathkey >>*= fun () ->
    Log.warn (fun f -> f "old file is now removed" ) ;
    Chamelon.set root pathkey (Cstruct.to_string newdata) >>*= fun () ->
    Log.warn (fun f -> f "new file is now created" ) ;
    Lwt.return_unit

So far the debug logs I got are (melt with mines, I put some comments below to help understanding):

2022-10-06 19:48:19 +02:00: DBG [sshfs_protocol] [SSH_FXP_WRITE 17] '/test.bin' @127 (512) <- this is the 2nd fwrite call (append data)

2022-10-06 19:48:19 +02:00: DBG [chamelon-fs] name "test.bin" is associated with id 4 on blockpair 0,
1
2022-10-06 19:48:19 +02:00: DBG [chamelon-fs] found 3 entries for id 4 in 0,
1
2022-10-06 19:48:19 +02:00: DBG [chamelon-fs] after compaction, there were 3 entries for id 4 in 0,
1
2022-10-06 19:48:19 +02:00: DBG [chamelon-fs] found 3 entries with name test.bin
2022-10-06 19:48:19 +02:00: WRN [sshfs_fs] got 127 bytes before offset 127 <- this is the log call after the first get_partial
2022-10-06 19:48:19 +02:00: DBG [chamelon-fs] getting size on key /test.bin
2022-10-06 19:48:19 +02:00: DBG [chamelon-fs] name "test.bin" is associated with id 4 on blockpair 0,
1
2022-10-06 19:48:19 +02:00: DBG [chamelon-fs] found 3 entries for id 4 in 0,
1
2022-10-06 19:48:19 +02:00: DBG [chamelon-fs] after compaction, there were 3 entries for id 4 in 0,
1
2022-10-06 19:48:19 +02:00: DBG [chamelon-fs] found 1 entries with name test.bin
2022-10-06 19:48:19 +02:00: DBG [chamelon-fs] name "test.bin" is associated with id 4 on blockpair 0,
1
2022-10-06 19:48:19 +02:00: DBG [chamelon-fs] found 3 entries for id 4 in 0,
1
2022-10-06 19:48:19 +02:00: DBG [chamelon-fs] after compaction, there were 3 entries for id 4 in 0,
1
2022-10-06 19:48:19 +02:00: DBG [chamelon-fs] found 3 entries with name test.bin <- and hangs

Don't hesitate to ask me for any other helpful information :)

[palainp]

Update: I just tried to show the value returned by the size call on the line [chamelon-fs] getting size on key /test.bin and it's 512 (the block size), which totally makes sense!
To explain a bit the situation, before the hangs I have test.bin (size=512B, usefull data=127B), I try to append some data at the end (512).

• I read from 0 to 127 (read_before)
• I read from 127+512 (=offset+new_data_length) to 512 (=size) : it hangs

I try with various append sized data, and it turns out that when the final size (real size + appending size) is above half a block it hangs :(