fix(graphql): normalize operation-root fragment selections

[tonisole]

Description

Normalize GraphQL operation-root fragments before exposing the selected fields
to query, mutation or subscription resolvers.

Why we encountered this

We encountered this while validating similarity queries in composed GraphQL
documents. A valid root named fragment caused an internal panic. The same
failure was reproduced with an ordinary generated get, so this is a general
operation-normalization issue, independent of similarity search.

This is a focused main-based correction. It does not include or depend on
#9837, #9840 or the separately submitted ById
rewriter correction.

Minimal reproduction

For a generated getBook query, send:

query($id: ID!, $enabled: Boolean!) {
  ...Books
}
fragment Books on Query {
  getBook(id: $id) @include(if: $enabled) {
    id
  }
}

Instead of resolving the valid operation, the original code panics:

interface conversion: ast.Selection is *ast.FragmentSpread, not *ast.Field
graphql/schema/request.go:90

Inline operation-root fragments have the same invalid cast.

Root cause and fix

Operation assumes every root selection is *ast.Field before expanding
nested fragments. Root fragment spreads and inline fragments are valid
selections but are not fields.

Use a synthetic root field with the actual Query, Mutation or Subscription
type and normalize it with the existing type-aware recursive collector.
This reuses directive evaluation, applicable type conditions, field grouping
and recursive expansion instead of adding a second fragment implementation.
Field order and merged aliases are preserved.

Review follow-up 62eccf1cd also checks that the selected operation's root
exists before constructing the synthetic field, and requires exactly one
subscription root field after collection. This rejects unsupported mutation/
subscription operations without panicking and prevents a single fragment
from hiding multiple subscription roots. Repeated compatible aliases inside
fragments still merge into one field; query/mutation cardinality is unchanged.

Regression coverage and validation

The regression extends the existing wrappers_test.go file using
testify/require, rather than adding a standalone test framework.

• Nine cases cover named/inline query roots, named mutation ordering,
  subscriptions, included/excluded fragments, selected operations with shared
  fragments, merged aliases/child fields and skipped inline fragments.
• Review follow-up adds 15 cases: six absent-root/selected-operation checks,
  eight subscription cardinality and valid alias/selection controls, and an
  explicit multi-field query control. Before the guards, three cases reproduce
  nil-root panics and four invalid subscriptions are incorrectly accepted.
• All 24 focused cases pass, together with
  go test -race -count=1 ./graphql/schema ./graphql/resolve,
  scoped vet, formatting and diff checks; the standalone main-based Dgraph
  binary builds with jemalloc. This follow-up does not replace any installed
  binary or claim a new combined downstream runtime certification.
• Regressions fail before the change and pass afterwards.
• The exact standalone main-based source passes
  go test -race -count=1 ./graphql/schema, go vet ./graphql/schema,
  formatting and git diff --check; the Dgraph binary builds.
• The combined candidate also passes isolated runtime ordinary-get/similarity
  fragment controls under both HS256 and RS256, and downstream multi-root
  and fragment/directive acceptance without skips. Application-specific
  fixtures are not part of this contribution.

This fixes valid existing GraphQL documents; it does not add a new API,
change authorization rules or alter the requested operation kind.

Checklist

• PR title follows Conventional Commits syntax.
• Standalone Dgraph source compiles.
• Existing schema runner passes with race detection.
• Formatting, diff checks and scoped vet pass.
• Full Trunk linting passes locally; Trunk is not installed in this environment.

---

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Summary by CodeRabbit

• Bug Fixes
  • Improved handling of named and inline fragments in GraphQL operations, including queries, mutations, and subscriptions. Directive rules and operation selection are respected, and fields with the same alias are merged.
  • Operations now return an error when their required schema root is undefined. Subscriptions return an error unless fragment-expanded selections resolve to exactly one top-level field.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: 1d0a193a-ce30-481e-bc73-d6f6ade5ee19

📥 Commits

Reviewing files that changed from the base of the PR and between 254ffaa and 62eccf1.

📒 Files selected for processing (2)

• graphql/schema/request.go
• graphql/schema/wrappers_test.go

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---

📝 Walkthrough

Walkthrough

Operation parsing now expands fragments from the operation root using the matching query, mutation, or subscription schema type. Tests cover root fragment selection and directives across these operation types.

Changes

Root fragment expansion

Layer / File(s)	Summary
Normalize operation root selections graphql/schema/request.go, graphql/schema/wrappers_test.go	Operation wraps the root selection set in a synthetic field that uses the selected schema root type, then applies type-aware fragment collection. Tests cover query, mutation, and subscription roots, including directives and merged selections.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: mlwelles

Merge Risk: ⚪ Minimal · up to 62ecc

The identified root-selection failures appear addressed, with no actionable merge-blocking risk established by the supplied evidence.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 254ff

The change remains within GraphQL request processing and preserves the existing authorization and failure-containment paths. Risk is low, with residual uncertainty around the end-to-end effects of grouping repeated mutation fields.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The directly affected scope is GraphQL operation construction across query, mutation, and subscription requests. Newly accepted root fragments can reach existing resolvers, including write-capable mutation resolvers, but the inspected path adds no new privilege source or cross-system dependency.

Trust Boundaries and Controls

• observed — Client-controlled documents and variables pass validation before collection. Collection evaluates skip/include directives and applicable fragment types using the selected schema root and resolved variables. Resolver dispatch and authorization remain downstream controls rather than being inferred from fragment contents.

Resilience and Maintainability Implications

• observed — The existing resolver recovery boundary surrounds Operation construction and converts remaining panics into response errors. This supports request-level containment; it does not establish transaction rollback or atomicity across multiple mutations.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: normalizing fragment selections at operation roots.

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Actionable comments posted: 2

---

• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.

Inline comments:
Review comments at @graphql/schema/request.go:
- Line 101: Update the operation handling around `op.SelectionSet =
root.SelectionSet` to validate subscription root-field cardinality after
expanding fragment spreads and before accepting the operation. Require exactly
one collected root field, including fields reached through nested fragments,
while leaving non-subscription handling unchanged.
- Line 97: In schema.Operation, validate that the selected operation’s rootType
is non-nil before constructing the synthetic root field; return an error when
the schema has no root type so the request is rejected rather than panicking.

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ℹ️ Review info

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: e0c4a493-a41c-406f-8fe1-117dd1fbeb1f

📥 Commits

Reviewing files that changed from the base of the PR and between b8236a7 and 254ffaa.

📒 Files selected for processing (2)

• graphql/schema/request.go
• graphql/schema/wrappers_test.go

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 9 remain after this review.