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The current implementation sets all the environment variables passed in Process.Env in the current process, one by one, then uses os.Environ to read those back. As pointed out in [1], this is slow, as runc calls os.Setenv for every variable, and there may be a few thousands of those. Looking into how os.Setenv is implemented, it is indeed slow, especially when cgo is enabled. Looking into why it was implemented the way it is, I found commit 9744d72 and traced it to [2], which discusses the actual reasons. It boils down to these two: - HOME is not passed into container as it is set in setupUser by os.Setenv and has no effect on config.Env; - there is a need to deduplication of environment variables. Yet it was decided in [2] to not go ahead with this patch, but later [3] was opened with the carry of this patch, and merged. Now, from what I see: 1. Passing environment to exec is way faster than using os.Setenv and os.Environ (tests show ~20x speed improvement in a simple Go test, and ~3x improvement in real-world test, see below). 2. Setting environment variables in the runc context may result is some ugly side effects (think GODEBUG, LD_PRELOAD, or _LIBCONTAINER_*). 3. Nothing in runtime spec says that the environment needs to be deduplicated, or the order of preference (whether the first or the last value of a variable with the same name is to be used). We should stick to what we have in order to maintain backward compatibility. So, this patch: - switches to passing env directly to exec; - adds deduplication mechanism to retain backward compatibility; - takes care to set PATH from process.Env in the current process (so that supplied PATH is used to find the binary to execute), also to retain backward compatibility; - adds HOME to process.Env if not set. The benchmark added by the previous commit shows ~3x improvement: │ before │ after │ │ sec/op │ sec/op vs base │ ExecInBigEnv-20 61.53m ± 1% 21.87m ± 16% -64.46% (p=0.000 n=10) [1]: #1983 [2]: docker-archive/libcontainer#418 [3]: docker-archive/libcontainer#432 Signed-off-by: Kir Kolyshkin <[email protected]>
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,59 @@ | ||
| package libcontainer | ||
|
|
||
| import ( | ||
| "errors" | ||
| "fmt" | ||
| "os" | ||
| "slices" | ||
| "strings" | ||
| ) | ||
|
|
||
| // prepareEnv processes a list of environment variables, preparing it | ||
| // for direct consumption by unix.Exec. In particular, it: | ||
| // - validates each variable is in the NAME=VALUE format and | ||
| // contains no \0 (nil) bytes; | ||
| // - removes any duplicates (keeping only the last value for each key) | ||
| // - sets PATH for the current process, if found in the list. | ||
| // | ||
| // It returns the deduplicated environment, a flag telling whether HOME | ||
| // is present in the input, and an error. | ||
| func prepareEnv(env []string) ([]string, bool, error) { | ||
| if env == nil { | ||
| return nil, false, nil | ||
| } | ||
| // Deduplication code based on dedupEnv from Go 1.22 os/exec. | ||
|
|
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| // Construct the output in reverse order, to preserve the | ||
| // last occurrence of each key. | ||
| out := make([]string, 0, len(env)) | ||
| saw := make(map[string]bool, len(env)) | ||
| for n := len(env); n > 0; n-- { | ||
| kv := env[n-1] | ||
| i := strings.IndexByte(kv, '=') | ||
| if i == -1 { | ||
| return nil, false, errors.New("invalid environment variable: missing '='") | ||
| } | ||
| if i == 0 { | ||
| return nil, false, errors.New("invalid environment variable: name cannot be empty") | ||
| } | ||
| key := kv[:i] | ||
| if saw[key] { // Duplicate. | ||
| continue | ||
| } | ||
| saw[key] = true | ||
| if strings.IndexByte(kv, 0) >= 0 { | ||
| return nil, false, fmt.Errorf("invalid environment variable %q: contains nul byte (\\x00)", key) | ||
| } | ||
| if key == "PATH" { | ||
| // Needs to be set as it is used for binary lookup. | ||
| if err := os.Setenv("PATH", kv[i+1:]); err != nil { | ||
| return nil, false, err | ||
| } | ||
| } | ||
| out = append(out, kv) | ||
| } | ||
| // Restore the original order. | ||
| slices.Reverse(out) | ||
|
|
||
| return out, saw["HOME"], nil | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,40 @@ | ||
| package libcontainer | ||
|
|
||
| import ( | ||
| "slices" | ||
| "testing" | ||
| ) | ||
|
|
||
| func TestPrepareEnvDedup(t *testing.T) { | ||
| tests := []struct { | ||
| env, wantEnv []string | ||
| }{ | ||
| { | ||
| env: []string{}, | ||
| wantEnv: []string{}, | ||
| }, | ||
| { | ||
| env: []string{"HOME=/root", "FOO=bar"}, | ||
| wantEnv: []string{"HOME=/root", "FOO=bar"}, | ||
| }, | ||
| { | ||
| env: []string{"A=a", "A=b", "A=c"}, | ||
| wantEnv: []string{"A=c"}, | ||
| }, | ||
| { | ||
| env: []string{"TERM=vt100", "HOME=/home/one", "HOME=/home/two", "TERM=xterm", "HOME=/home/three", "FOO=bar"}, | ||
| wantEnv: []string{"TERM=xterm", "HOME=/home/three", "FOO=bar"}, | ||
| }, | ||
| } | ||
|
|
||
| for _, tc := range tests { | ||
| env, _, err := prepareEnv(tc.env) | ||
| if err != nil { | ||
| t.Error(err) | ||
| continue | ||
| } | ||
| if !slices.Equal(env, tc.wantEnv) { | ||
| t.Errorf("want %v, got %v", tc.wantEnv, env) | ||
| } | ||
| } | ||
| } |
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