pkg/util/rand helpers panic on invalid input instead of returning errors

[hazelmayank]

Describe the bug

pkg/util/rand provides two helpers — String and StringFromCharset — that both return (string, error). The signature implies invalid inputs should surface as a Go error, but some invalid inputs currently cause the process to panic instead:

• StringFromCharset(1, "") panics in crypto/rand.Int because the upper bound is <= 0.
• String(-1) panics in make([]byte, -1) because the slice length is negative.

Since both functions already declare an error return, callers naturally expect that path to be used. Today they cannot recover — the helpers crash the host process before any caller code can react.

---

Expected behavior

Invalid inputs should return a clear error instead of panicking:

StringFromCharset(1, "")  ->  "", error("rand: cannot generate ... from empty charset")
String(-1)                ->  "", error("rand: requested length -1 is negative")

n == 0 should keep returning ("", nil) regardless of the charset, since that's the natural no-op result for both helpers.

---

Actual behavior

StringFromCharset(1, "") panics with:

panic: crypto/rand: argument to Int is <= 0

String(-1) panics with:

panic: runtime error: makeslice: len out of range

In both cases the process crashes; the error return value is never set.

---

How to Reproduce?

Reproduced locally on the latest master.

Reproducer for empty charset:

package main

import (
    "fmt"

    r "github.com/microcks/microcks-cli/pkg/util/rand"
)

func main() {
    fmt.Println("Testing empty charset...")
    value, err := r.StringFromCharset(1, "")
    fmt.Printf("value=%q err=%v\n", value, err)
}

Output:

Testing empty charset...
panic: crypto/rand: argument to Int is <= 0

Reproducer for negative length:

package main

import (
    "fmt"

    r "github.com/microcks/microcks-cli/pkg/util/rand"
)

func main() {
    fmt.Println("Testing negative length...")
    value, err := r.String(-1)
    fmt.Printf("value=%q err=%v\n", value, err)
}

Output:

Testing negative length...
panic: runtime error: makeslice: len out of range

In both cases the program exits via panic before the err= line can print.

---

Microcks version or git rev

No response

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Install method (docker-compose, helm chart, operator, docker-desktop extension,...)

No response

---

Additional information

Root cause. StringFromCharset in pkg/util/rand/rand.go does not validate n or charset before allocating the output slice and calling crypto/rand.Int:

func StringFromCharset(n int, charset string) (string, error) {
    b := make([]byte, n)
    maxIdx := big.NewInt(int64(len(charset)))
    for i := 0; i < n; i++ {
        randIdx, err := rand.Int(rand.Reader, maxIdx)
        ...
    }
    ...
}

• n < 0 → make([]byte, n) panics with makeslice: len out of range.
• n > 0 with charset == "" → rand.Int(rand.Reader, big.NewInt(0)) panics with argument to Int is <= 0.
• n == 0 → already works correctly (loop doesn't execute, returns ("", nil)).

String calls StringFromCharset and inherits the negative-length panic.

Proposed fix. Add two guard clauses at the top of StringFromCharset:

if n < 0 {
    return "", fmt.Errorf("rand: requested length %d is negative", n)
}
if n > 0 && len(charset) == 0 {
    return "", fmt.Errorf("rand: cannot generate %d-character string from empty charset", n)
}

This preserves every currently-working input (including n == 0) and turns both panic paths into descriptive errors. The public signatures stay the same, so no callers (including the SSO PKCE flow in cmd/login.go) need to change.

A small test file pkg/util/rand/rand_test.go would lock this in with table-driven cases for both valid and invalid inputs — the package currently has no test coverage.

I would like to send a small PR for this if the maintainers agree on the approach.

[hazelmayank]

Opened a PR that adds input validation and unit tests for the invalid input panic cases in pkg/util/rand.