work (nix-shell wrapper)

work.c

#define _XOPEN_SOURCE 700

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/stat.h>

extern char **environ;
static char *cfgdirs[] = {
    "/.nix-profile/cfg/work/",
    "/lib/work/",
    NULL,
};
static char *nixdirs[] = {
    "/nixpkgs", // First one is tried without the workspace appended.
    "/nixpkgs-",
    NULL,
};

char *workdir(char *n) {
    char *d;
    struct stat s;
    size_t sz;

    d = getenv("WORK_DIR");
    if(d != NULL)
        return d;
    sz = 7 + strlen(getenv("HOME")) + strlen(n);
    d = malloc(sz);
    snprintf(d, sz,"%s/work/%s", getenv("HOME"), n);
    if(stat(d, &s) != 0)
        mkdir(d, 0755); // Don't care if this fails.
    return d;
}

char *workcfgdir() {
    char *d;
    int i;
    size_t sz;
    struct stat s;

    d = getenv("WORK_CFG_DIR");
    if(d != NULL)
        return d;
    sz = 24 + strlen(getenv("HOME"));
    d = malloc(sz);
    strncpy(d, getenv("HOME"), sz);
    for(i = 0; cfgdirs[i] != NULL; i++){
        strcat(d, cfgdirs[i]);
        if(stat(d, &s) == 0){
            break;
        }
    }
    if(cfgdirs[i] == NULL){
        perror("no config dirs exist");
        exit(EXIT_FAILURE);
    }
    return d;
}

char *nixpath(char *wd, char *n) {
    int i;
    char *np, d[4096];
    struct stat s;

    np = malloc(4096);
    strcpy(np, getenv("NIX_PATH"));
    for(i = 0; nixdirs[i] != NULL; i++, d[0] = '\0'){
        strcat(d, wd);
        strcat(d, nixdirs[i]);
        if(i != 0)
            strcat(d, n);
        if(stat(d, &s) == 0){
            strcat(np, ":");
            strcat(np, n);
            strcat(np, "=");
            strcat(np, d);
            break;
        }
    }
    strcat(np, ":ssh-config-file=/dev/null:ssh-auth-sock=/dev/null");
    return np;
}

int main(int argc, char *argv[]) {
    char *wd, *wcd, *np;

    if(argc != 2){
        fprintf(stderr, "specify work directory\n");
        exit(EXIT_FAILURE);
    }

    wd = workdir(argv[1]);
    if(setenv("WORK_DIR", wd, 1) != 0){
        perror("unable to set environment");
        exit(EXIT_FAILURE);
    }
    printf("# work dir: %s\n", wd);

    wcd = workcfgdir();
    if(setenv("WORK_CFG_DIR", wcd, 1) != 0){
        perror("unable to set environment");
        exit(EXIT_FAILURE);
    }
    printf("# config dir: %s\n", wcd);

    np = nixpath(wd, argv[1]);
    if(setenv("NIX_PATH", np, 1) != 0){
        perror("unable to set environment");
        exit(EXIT_FAILURE);
    }
    printf("# using NIX_PATH: %s\n", np);

    char *sh;
    sh = getenv("SHELL");
    if(sh == NULL)
        sh = "bash";
    int sz;
    sz = 5 + strlen(argv[1]) + strlen(wcd);
    char nf[sz];
    snprintf(nf, sz, "%s%s.nix", wcd, argv[1]);

    execlp("nix-shell", "--show-trace", "--command", sh, nf, NULL);
}