fix: guard against empty Build.Image in ImageNameWithDigest

[Elvand-Lie]

Description

ImageNameWithDigest in pkg/functions/function.go does not validate that Build.Image is non-empty before processing. When called with a non-empty newDigest but an empty image name, it produces the malformed OCI reference @sha256:... (missing the image name prefix), causing confusing downstream name.ParseReference errors.

Added early return for empty image.

Changes

• pkg/functions/function.go, line 866: added if image == "" { return "" } guard

Reproduction

f := fn.Function{Build: fn.BuildSpec{Image: ""}}
result := f.ImageNameWithDigest("sha256:abc123")
// Before fix: "@sha256:abc123" (invalid OCI reference)
// After fix:  "" (safe empty string)

Verification

• go vet ./pkg/functions: clean
• go test ./pkg/functions/ -run TestFunction_ImageWithDigest -count=1: all 5 tests pass

Fixes #3902

[knative-prow]

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[knative-prow]

Hi @Elvand-Lie. Thanks for your PR.

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[Elvand-Lie]

Verification Evidence

1. go vet: clean

$ go vet ./pkg/functions
# (no output, clean)

2. Before vs After

f := fn.Function{Build: fn.BuildSpec{Image: ""}}

// BEFORE fix:
f.ImageNameWithDigest("sha256:abc123")
// produces "@sha256:abc123", which is an INVALID OCI reference (no image name before @)
// Downstream: name.ParseReference("@sha256:abc123") fails with "could not parse reference"

// AFTER fix:
f.ImageNameWithDigest("sha256:abc123")
// produces "", a safe empty string that callers can check and handle gracefully

3. Code path trace showing how you hit this bug

client.go:Push()
  calls pusher.Push(), which returns imageDigest
  then client.go:1275 sets f.Build.Image = f.ImageNameWithDigest(imageDigest)
    if Build.Image was "" (interrupted build, corrupt func.yaml),
    this produces "@sha256:..." which fails all subsequent operations

4. Existing test suite, all pass

$ go test ./pkg/functions/ -run TestFunction_ImageWithDigest -count=1 -v
=== RUN   TestFunction_ImageWithDigest
=== RUN   TestFunction_ImageWithDigest/Full_path_with_port
=== RUN   TestFunction_ImageWithDigest/Path_with_namespace
=== RUN   TestFunction_ImageWithDigest/Just_image_name
=== RUN   TestFunction_ImageWithDigest/Full_path_with_port_and_SHA256_Digest
=== RUN   TestFunction_ImageWithDigest/Full_path_with_port_and_SHA256_Digest_with_empty_ImageDigest
--- PASS: TestFunction_ImageWithDigest (0.00s)
PASS
ok  	knative.dev/func/pkg/functions	0.394s

5. The fix (3 lines)

 image := f.Build.Image
+if image == "" {
+    return ""
+}

Minimal and defensive. No behavior change for any non-empty image path.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.68%. Comparing base (f72ecda) to head (03daf3f).

Files with missing lines	Patch %	Lines
pkg/functions/function.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3905      +/-   ##
==========================================
- Coverage   53.93%   51.68%   -2.25%     
==========================================
  Files         200      200              
  Lines       23681    23682       +1     
==========================================
- Hits        12772    12240     -532     
- Misses       9674    10301     +627     
+ Partials     1235     1141      -94     

Flag	Coverage Δ
e2e	33.42% <0.00%> (-0.01%)	⬇️
e2e go	29.38% <0.00%> (-0.01%)	⬇️
e2e node	25.70% <0.00%> (-0.01%)	⬇️
e2e python	29.71% <0.00%> (-0.01%)	⬇️
e2e quarkus	25.83% <0.00%> (-0.01%)	⬇️
e2e rust	25.30% <0.00%> (-0.01%)	⬇️
e2e springboot	?
e2e typescript	25.82% <0.00%> (-0.01%)	⬇️
e2e-config-ci	26.90% <0.00%> (+<0.01%)	⬆️
integration	?
unit macos-14	43.04% <0.00%> (-0.01%)	⬇️
unit macos-latest	43.04% <0.00%> (-0.01%)	⬇️
unit ubuntu-24.04-arm	43.34% <0.00%> (-0.02%)	⬇️
unit ubuntu-latest	43.89% <0.00%> (-0.01%)	⬇️
unit windows-latest	43.10% <0.00%> (-0.01%)	⬇️

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