Add reproduction for cargo pgrx install --config (issue #2135) for reproducing issue.

Author: isdaniel

repro/install-config-2135/README.md

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+# Reproduction: why `cargo pgrx install --config` is needed (issue #2135)
+
+A self-contained, Docker-based reproduction of the real-world scenario PR #2343
+fixes: a pgrx extension that depends on an **internal crate from a private
+registry**, in a monorepo where the registry is declared only in the
+extension's own `.cargo/config.toml`.
+
+## Why it's real
+
+Companies run a **private registry** (Artifactory, ktra, a crates.io mirror, …)
+for internal crates, declared per-repo in `.cargo/config.toml`. A pgrx extension
+lives as one submodule/subfolder of a larger monorepo. Cargo resolves
+`.cargo/config.toml` **from the current directory upward**, so building the
+extension from the monorepo root inherits the *parent* config — which does not
+know the private registry — and `cargo pgrx install` dies at its first step,
+`cargo metadata`. `cargo build` lets you fix this with `--config`; this PR gives
+`cargo pgrx install` the same.
+
+## Layout (everything is a real file — nothing is generated inside the script)
+
+```
+repro/install-config-2135/
+├── repro.sh                                  # thin orchestrator (build/run/publish/before/after)
+├── registry/
+│   └── Dockerfile                            # the private registry image (cargo-http-registry)
+└── files/
+    ├── acme-internal/                        # the internal crate, published to the registry
+    │   ├── Cargo.toml
+    │   ├── src/lib.rs                         #   pub fn secret() -> i32 { 42 }
+    │   └── .cargo/config.toml                 #   registry def, for `cargo publish`
+    └── monorepo/
+        ├── .cargo/config.toml                # PARENT config — does NOT define `acme`
+        └── childext/                         # the pgrx extension
+            ├── Cargo.toml                     #   depends on acme-internal { registry = "acme" }
+            ├── childext.control
+            ├── src/lib.rs                     #   #[pg_extern] fn acme_secret() -> acme_internal::secret()
+            └── .cargo/config.toml             #   CHILD config — the ONLY place `acme` is defined
+```
+
+`repro.sh` copies `files/` into a scratch dir (`/tmp/acme-demo`) to build, so the
+nested crates never interfere with the pgrx workspace.
+
+## Run
+
+```bash
+CARGO_PGRX=target/debug/cargo-pgrx \         # the PR build of cargo-pgrx
+PG_CONFIG=$HOME/.pgrx/18.4/pgrx-install/bin/pg_config \
+PG_PORT=5449 \                               # pgrx base_port + major (5431 + 18)
+PGRX_SRC=$PWD \                              # your pgrx checkout
+repro/install-config-2135/repro.sh
+```
+
+Expected:
+
+- **BEFORE** (no `--config`):
+  `cargo metadata ... registry index was not found in any configuration: 'acme'`
+- **AFTER** (`--config childext/.cargo/config.toml`): builds + installs.
+- **SQL proof**: `SELECT acme_secret();` → `42`.
+
+Teardown: `docker rm -f acme-registry && rm -rf /tmp/acme-demo`
+
+## Notes / honest caveats
+
+- **`PGRX_BUILD_FLAGS` can't fix this** — it only reaches the build step, never
+  the `cargo metadata` call that runs first (and fails first). The PR forwards
+  `--config` to every cargo invocation install makes (metadata, rustc, the
+  `get_version` metadata, and the schema `cargo run`).
+- **Registry index over `file://`** — `cargo-http-registry`'s HTTP git-index does
+  not serve cleanly in every environment (cargo can get HTTP 500), so the index
+  is read from the registry's git repo on the shared Docker volume, while publish
+  + crate downloads still go over HTTP to the container. What's being demonstrated
+  (config resolution / `--config`) is unaffected.
+- **`CARGO_TARGET_DIR` is set** by `repro.sh`: `pgrx-pg-config::get_target_dir()`
+  runs its own `cargo metadata --no-deps`, which an undefined registry also breaks
+  and which does not yet receive `--config` (gap #4). Setting `CARGO_TARGET_DIR`
+  makes it return early, as pgrx/CI usually do. Forwarding `--config` there too is
+  a possible follow-up.

repro/install-config-2135/files/acme-internal/.cargo/config.toml

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+# Registry definition used when PUBLISHING acme-internal.
+# The index is the registry's git repo on the shared Docker volume; publish +
+# downloads go over HTTP to the container (see registry/Dockerfile).
+[registries.acme]
+index = "file:///tmp/acme-demo/reg/.git"

repro/install-config-2135/files/acme-internal/Cargo.toml

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+[package]
+name = "acme-internal"
+version = "0.1.0"
+edition = "2021"
+description = "Internal-only crate, available ONLY from the private `acme` registry"
+license = "MIT"
+
+[dependencies]

repro/install-config-2135/files/acme-internal/src/lib.rs

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+//! A pretend company-internal crate. It is published only to the private
+//! `acme` registry, so anything depending on it can only build when that
+//! registry is configured.
+
+/// The "secret" value the extension will expose through SQL.
+pub fn secret() -> i32 {
+    42
+}

repro/install-config-2135/files/monorepo/.cargo/config.toml

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+# monorepo-root config, shared across the org's many Rust services.
+# NOTE: it does NOT define the `acme` registry — that lives in the extension's own childext/.cargo/config.toml. Building from this root inherits THIS file.
+[net]
+retry = 2

repro/install-config-2135/files/monorepo/childext/.cargo/config.toml

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+# This submodule's OWN config. It declares the private `acme` registry that `childext` depends on. The parent monorepo config (../.cargo/config.toml) does not have this, so building from the monorepo root cannot see it — which is the whole point: you must pass `--config childext/.cargo/config.toml` so cargo (and `cargo pgrx install`) picks up the right config for this repo.
+[target.'cfg(target_os="macos")']
+rustflags = ["-Clink-arg=-Wl,-undefined,dynamic_lookup"]
+
+[registries.acme]
+index = "file:///tmp/acme-demo/reg/.git"

repro/install-config-2135/files/monorepo/childext/Cargo.toml

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+[package]
+name = "childext"
+version = "0.0.0"
+edition = "2021"
+
+[lib]
+crate-type = ["cdylib"]
+
+[features]
+default = ["pg13"]
+pg13 = ["pgrx/pg13", "pgrx-tests/pg13"]
+pg14 = ["pgrx/pg14", "pgrx-tests/pg14"]
+pg15 = ["pgrx/pg15", "pgrx-tests/pg15"]
+pg16 = ["pgrx/pg16", "pgrx-tests/pg16"]
+pg17 = ["pgrx/pg17", "pgrx-tests/pg17"]
+pg18 = ["pgrx/pg18", "pgrx-tests/pg18"]
+pg19 = ["pgrx/pg19", "pgrx-tests/pg19"]
+pg_test = []
+
+[dependencies]
+acme-internal = { version = "0.1", registry = "acme" }
+pgrx = { path = "__PGRX_SRC__/pgrx" }
+
+[dev-dependencies.pgrx-tests]
+path = "__PGRX_SRC__/pgrx-tests"
+
+[profile.dev]
+panic = "unwind"
+
+[profile.release]
+panic = "unwind"
+opt-level = 3
+lto = "fat"
+codegen-units = 1

repro/install-config-2135/files/monorepo/childext/childext.control

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+comment = 'childext: demonstrates a pgrx extension depending on a private-registry crate'
+default_version = '@CARGO_VERSION@'
+module_pathname = 'childext'
+relocatable = false
+superuser = true
+trusted = false

repro/install-config-2135/files/monorepo/childext/src/lib.rs

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+use pgrx::prelude::*;
+
+::pgrx::pg_module_magic!();
+
+/// Exposes a value that comes from the private `acme-internal` crate, proving
+/// the internal dependency was resolved, compiled and linked into the extension.
+#[pg_extern]
+fn acme_secret() -> i32 {
+    acme_internal::secret()
+}

repro/install-config-2135/registry/Dockerfile

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+FROM rust:slim
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends git ca-certificates pkg-config libssl-dev \
+    && rm -rf /var/lib/apt/lists/* \
+    && cargo install cargo-http-registry \
+    && git config --system user.email "registry@acme.invalid" \
+    && git config --system user.name  "acme registry" \
+    && git config --system --add safe.directory '*'
+EXPOSE 8444
+# /reg is a bind-mounted volume holding the git index + uploaded .crate files
+ENTRYPOINT ["cargo-http-registry", "/reg", "--addr", "0.0.0.0:8444"]